diff --git a/CHANGELOG.md b/CHANGELOG.md
index e5d5795e..f54cbc3d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Changelog
 
+# 8.8.1 (unreleased)
+
+- Remove support for generating legacy (not null safe) code.
+
 # 8.8.0
 
 - Allow classes with record fields to be serialized if they use a typedef for
diff --git a/benchmark/pubspec.yaml b/benchmark/pubspec.yaml
index 9c325d56..a82b8a58 100644
--- a/benchmark/pubspec.yaml
+++ b/benchmark/pubspec.yaml
@@ -6,11 +6,11 @@ description: >
 homepage: https://github.com/google/built_value.dart
 
 environment:
-  sdk: '>=2.12.0 <4.0.0'
+  sdk: '>=3.0.0 <4.0.0'
 
 dependencies:
   built_collection: ^5.0.0
-  built_value: ^8.7.0
+  built_value: ^8.8.0
 
 dev_dependencies:
   build_runner: '>=1.0.0 <3.0.0'
diff --git a/built_value/pubspec.yaml b/built_value/pubspec.yaml
index ead79557..7f1a70a8 100644
--- a/built_value/pubspec.yaml
+++ b/built_value/pubspec.yaml
@@ -8,7 +8,7 @@ topics:
  - built-value
 
 environment:
-  sdk: '>=2.12.0 <4.0.0'
+  sdk: '>=3.0.0 <4.0.0'
 
 dependencies:
   built_collection: ^5.0.0
diff --git a/built_value_generator/lib/src/serializer_source_class.dart b/built_value_generator/lib/src/serializer_source_class.dart
index f35ec2d1..d806b2b5 100644
--- a/built_value_generator/lib/src/serializer_source_class.dart
+++ b/built_value_generator/lib/src/serializer_source_class.dart
@@ -78,15 +78,6 @@ abstract class SerializerSourceClass
   @memoized
   String get name => element.name;
 
-  @memoized
-  bool get isNonNullByDefault => element.library.isNonNullableByDefault;
-
-  @memoized
-  String get orNull => isNonNullByDefault ? '?' : '';
-
-  @memoized
-  String get notNull => isNonNullByDefault ? '!' : '';
-
   @memoized
   String get wireName {
     if (isBuiltValue) {
@@ -133,7 +124,7 @@ abstract class SerializerSourceClass
       ? ''
       : '<' +
           genericBounds
-              .map((bound) => bound.isEmpty ? 'Object$orNull' : bound)
+              .map((bound) => bound.isEmpty ? 'Object?' : bound)
               .join(', ') +
           '>';
 
@@ -328,18 +319,18 @@ class $serializerImplName implements StructuredSerializer<$genericName> {
   final String wireName = '${escapeString(wireName)}';
 
   @override
-  Iterable<Object$orNull> serialize(Serializers serializers, $genericName object,
+  Iterable<Object?> serialize(Serializers serializers, $genericName object,
       {FullType specifiedType = FullType.unspecified}) {
-    ${fields.isEmpty ? 'return <Object$orNull>[];' : '''
+    ${fields.isEmpty ? 'return <Object?>[];' : '''
     ${_generateGenericsSerializerPreamble()}
-    final result = <Object$orNull>[${_generateRequiredFieldSerializers()}];
+    final result = <Object?>[${_generateRequiredFieldSerializers()}];
     ${_generateNullableFieldSerializers()}
     return result;
     '''}
   }
 
   @override
-  $genericName deserialize(Serializers serializers, Iterable<Object$orNull> serialized,
+  $genericName deserialize(Serializers serializers, Iterable<Object?> serialized,
       {FullType specifiedType = FullType.unspecified}) {
     ${_generateGenericsSerializerPreamble()}
     $maybeCastFn
@@ -348,9 +339,9 @@ class $serializerImplName implements StructuredSerializer<$genericName> {
 
     final iterator = serialized.iterator;
     while (iterator.moveNext()) {
-      final key = iterator.current$notNull as String;
+      final key = iterator.current! as String;
       iterator.moveNext();
-      final Object$orNull value = iterator.current;
+      final Object? value = iterator.current;
       switch (key) {
         ${_generateFieldDeserializers()}
       }
@@ -490,7 +481,7 @@ class $serializerImplName implements PrimitiveSerializer<$genericName> {
     var nullableFields = fields.where((field) => field.isNullable).toList();
     if (nullableFields.isEmpty) return '';
 
-    return 'Object$orNull value;' +
+    return 'Object? value;' +
         nullableFields.map((field) {
           var serializeField = '''serializers.serialize(
           value,
@@ -526,7 +517,7 @@ class $serializerImplName implements PrimitiveSerializer<$genericName> {
           compilationUnit, genericParameters.toBuiltSet());
       final cast = field.generateCast(compilationUnit, _genericBoundsAsMap);
       // If cast exists and is not nullable.
-      var maybeNotNull = !field.isNullable && cast.isNotEmpty ? notNull : '';
+      var maybeNotNull = !field.isNullable && cast.isNotEmpty ? '!' : '';
       if (field.builderFieldUsesNestedBuilder) {
         if (hasBuilder && field.builderFieldIsNullable) {
           // The manually implemented builder might or might not return a
@@ -536,7 +527,7 @@ class $serializerImplName implements PrimitiveSerializer<$genericName> {
 case '${escapeString(field.wireName)}':
   var maybeBuilder = result.${field.name};
   var fieldValue = serializers.deserialize(
-      value, specifiedType: $fullType)$notNull $cast;
+      value, specifiedType: $fullType)! $cast;
   if (maybeBuilder == null) {
     result.${field.name} = \$cast(fieldValue.toBuilder());
   } else {
@@ -552,7 +543,7 @@ case '${escapeString(field.wireName)}':
           return '''
 case '${escapeString(field.wireName)}':
   result.${field.name}.replace(serializers.deserialize(
-      value, specifiedType: $fullType)$notNull $looseCast);
+      value, specifiedType: $fullType)! $looseCast);
   break;
 ''';
         } else {
@@ -565,7 +556,7 @@ case '${escapeString(field.wireName)}':
         }
       } else {
         // `cast` is empty if no cast is needed.
-        var maybeOrNull = field.isNullable && cast.isNotEmpty ? orNull : '';
+        var maybeOrNull = field.isNullable && cast.isNotEmpty ? '?' : '';
         return '''
 case '${escapeString(field.wireName)}':
   result.${field.name} = serializers.deserialize(
diff --git a/built_value_generator/lib/src/serializer_source_class.g.dart b/built_value_generator/lib/src/serializer_source_class.g.dart
index b7b0e584..83d4f5ca 100644
--- a/built_value_generator/lib/src/serializer_source_class.g.dart
+++ b/built_value_generator/lib/src/serializer_source_class.g.dart
@@ -17,9 +17,6 @@ class _$SerializerSourceClass extends SerializerSourceClass {
   BuiltValueSerializer? __serializerSettings;
   BuiltValueEnum? __enumClassSettings;
   String? __name;
-  bool? __isNonNullByDefault;
-  String? __orNull;
-  String? __notNull;
   String? __wireName;
   String? __serializerDeclaration;
   BuiltList<String>? __genericParameters;
@@ -68,16 +65,6 @@ class _$SerializerSourceClass extends SerializerSourceClass {
   @override
   String get name => __name ??= super.name;
 
-  @override
-  bool get isNonNullByDefault =>
-      __isNonNullByDefault ??= super.isNonNullByDefault;
-
-  @override
-  String get orNull => __orNull ??= super.orNull;
-
-  @override
-  String get notNull => __notNull ??= super.notNull;
-
   @override
   String get wireName => __wireName ??= super.wireName;
 
diff --git a/built_value_generator/lib/src/serializer_source_field.dart b/built_value_generator/lib/src/serializer_source_field.dart
index a9148e40..2739f207 100644
--- a/built_value_generator/lib/src/serializer_source_field.dart
+++ b/built_value_generator/lib/src/serializer_source_field.dart
@@ -73,12 +73,6 @@ abstract class SerializerSourceField
             annotation.getField('autoCreateNestedBuilder')?.toBoolValue());
   }
 
-  @memoized
-  bool get isNonNullByDefault => element.library.isNonNullableByDefault;
-
-  @memoized
-  String get orNull => isNonNullByDefault ? '?' : '';
-
   @memoized
   bool get hasNullableAnnotation => element.getter!.metadata
       .any((metadata) => metadataToStringValue(metadata) == 'nullable');
@@ -168,7 +162,6 @@ abstract class SerializerSourceField
 
   @memoized
   bool get builderFieldIsNullable {
-    if (!isNonNullByDefault) return true;
     if (!builderFieldElementIsValid) return true;
 
     return fullBuildElementType == 'dynamic' ||
@@ -288,7 +281,7 @@ abstract class SerializerSourceField
         DartTypes.isBuiltCollectionTypeName(bareType) &&
         builderFieldUsesNestedBuilder) {
       if (bareType == 'BuiltList' || bareType == 'BuiltSet') {
-        generics = 'Object$orNull';
+        generics = 'Object?';
       } else if (bareType == 'BuiltMap' ||
           bareType == 'BuiltListMultimap' ||
           bareType == 'BuiltSetMultimap') {
diff --git a/built_value_generator/lib/src/serializer_source_field.g.dart b/built_value_generator/lib/src/serializer_source_field.g.dart
index 745834af..eae7d0b0 100644
--- a/built_value_generator/lib/src/serializer_source_field.g.dart
+++ b/built_value_generator/lib/src/serializer_source_field.g.dart
@@ -17,8 +17,6 @@ class _$SerializerSourceField extends SerializerSourceField {
   final FieldElement? builderElement;
   bool? __isSerializable;
   BuiltValueField? __builtValueField;
-  bool? __isNonNullByDefault;
-  String? __orNull;
   bool? __hasNullableAnnotation;
   bool? __hasNullableType;
   bool? __isNullable;
@@ -61,13 +59,6 @@ class _$SerializerSourceField extends SerializerSourceField {
   BuiltValueField get builtValueField =>
       __builtValueField ??= super.builtValueField;
 
-  @override
-  bool get isNonNullByDefault =>
-      __isNonNullByDefault ??= super.isNonNullByDefault;
-
-  @override
-  String get orNull => __orNull ??= super.orNull;
-
   @override
   bool get hasNullableAnnotation =>
       __hasNullableAnnotation ??= super.hasNullableAnnotation;
diff --git a/built_value_generator/lib/src/value_source_class.dart b/built_value_generator/lib/src/value_source_class.dart
index 8930c144..eb82a18f 100644
--- a/built_value_generator/lib/src/value_source_class.dart
+++ b/built_value_generator/lib/src/value_source_class.dart
@@ -52,18 +52,6 @@ abstract class ValueSourceClass
       ? 'mixin class'
       : 'class';
 
-  @memoized
-  bool get isNonNullByDefault => element.library.isNonNullableByDefault;
-
-  @memoized
-  String get orNull => isNonNullByDefault ? '?' : '';
-
-  @memoized
-  String get notNull => isNonNullByDefault ? '!' : '';
-
-  @memoized
-  String get late => isNonNullByDefault ? 'late ' : '';
-
   /// Returns the class name for the generated implementation. If the manually
   /// maintained class is private then we ignore the underscore here, to avoid
   /// returning a class name starting `_$_`.
@@ -623,7 +611,7 @@ abstract class ValueSourceClass
       if (!valueClassFactories.any(
           (factory) => factory.toSource().contains('$implName$_generics'))) {
         final exampleFactory = 'factory $name('
-            '[void Function(${name}Builder$_generics)$orNull updates]) = '
+            '[void Function(${name}Builder$_generics)? updates]) = '
             '$implName$_generics;';
         result.add(GeneratorError((b) => b
           ..message =
@@ -757,12 +745,11 @@ abstract class ValueSourceClass
     for (var field in fields) {
       final type = field.typeInCompilationUnit(compilationUnit);
       result.writeln('@override');
-      result.writeln(
-          'final $type${field.isNullable ? orNull : ''} ${field.name};');
+      result
+          .writeln('final $type${field.isNullable ? '?' : ''} ${field.name};');
     }
     for (var memoizedGetter in memoizedGetters) {
-      result.writeln(
-          '${memoizedGetter.returnType}$orNull __${memoizedGetter.name};');
+      result.writeln('${memoizedGetter.returnType}? __${memoizedGetter.name};');
       if (memoizedGetter.isNullable) {
         // Nullable memoiozed getters needs a field to store whether they are
         // initialized.
@@ -777,12 +764,12 @@ abstract class ValueSourceClass
     // to cast.
     if (hasBuilder) {
       result.writeln('factory $implName(['
-          'void Function(${name}Builder$_generics)$orNull updates]) '
+          'void Function(${name}Builder$_generics)? updates]) '
           '=> (new ${name}Builder$_generics()..update(updates)).build()'
           ' as $implName$_generics;');
     } else {
       result.writeln('factory $implName(['
-          'void Function(${name}Builder$_generics)$orNull updates]) '
+          'void Function(${name}Builder$_generics)? updates]) '
           '=> (new ${name}Builder$_generics()..update(updates))._build();');
     }
 
@@ -793,8 +780,7 @@ abstract class ValueSourceClass
     } else {
       result.write('$implName._({');
       result.write(fields.map((field) {
-        var maybeRequired =
-            (isNonNullByDefault && !field.isNullable) ? 'required ' : '';
+        var maybeRequired = field.isNullable ? '' : 'required ';
         return '${maybeRequired}this.${field.name}';
       }).join(', '));
       result.write('}) : super._()');
@@ -901,7 +887,7 @@ abstract class ValueSourceClass
     }
 
     // Builder holds a reference to a value, copies from it lazily.
-    result.writeln('$implName$_generics$orNull _\$v;');
+    result.writeln('$implName$_generics? _\$v;');
     result.writeln('');
 
     for (var field in fields) {
@@ -919,18 +905,16 @@ abstract class ValueSourceClass
 
       final isNullableNestedBuilder = field.isAutoCreateNestedBuilder &&
           field.isNestedBuilder &&
-          (!hasBuilder ||
-              field.builderElementSetterIsNullable ||
-              !isNonNullByDefault);
+          (!hasBuilder || field.builderElementSetterIsNullable);
       final hasNullableSetter = !hasBuilder ||
           isNullableNestedBuilder ||
           field.builderElementSetterIsNullable;
       final getterMaybeOrNull = hasNullableSetter &&
               (!field.isNestedBuilder || !field.isAutoCreateNestedBuilder)
-          ? orNull
+          ? '?'
           : '';
-      final setterMaybeOrNull = hasNullableSetter ? orNull : '';
-      final maybeLate = !hasNullableSetter ? late : '';
+      final setterMaybeOrNull = hasNullableSetter ? '?' : '';
+      final maybeLate = !hasNullableSetter ? 'late ' : '';
 
       late String trackedVariable;
 
@@ -1053,7 +1037,7 @@ abstract class ValueSourceClass
 
     result.writeln('@override');
     result.writeln(
-        'void update(void Function(${name}Builder$_generics)$orNull updates) {'
+        'void update(void Function(${name}Builder$_generics)? updates) {'
         ' if (updates != null) updates(this); }');
     result.writeln();
 
@@ -1138,7 +1122,7 @@ abstract class ValueSourceClass
       // in a nested builder then throw with more information. Otherwise,
       // just rethrow.
       result.writeln('} catch (_) {');
-      result.writeln('${late}String _\$failedField;');
+      result.writeln('late String _\$failedField;');
       result.writeln('try {');
       result.write(fieldBuilders.keys.map((field) {
         final fieldBuilder = fieldBuilders[field];
@@ -1202,7 +1186,7 @@ abstract class ValueSourceClass
     var generateMemoizedHashCode =
         declaresMemoizedHashCode && comparedFields.isNotEmpty;
     if (generateMemoizedHashCode) {
-      result.writeln('int$orNull __hashCode;');
+      result.writeln('int? __hashCode;');
     }
 
     result.writeln('@override');
@@ -1212,7 +1196,7 @@ abstract class ValueSourceClass
       result.writeln('return ${name.hashCode};');
     } else {
       if (generateMemoizedHashCode) {
-        result.writeln('if (__hashCode != null) return __hashCode$notNull;');
+        result.writeln('if (__hashCode != null) return __hashCode!;');
       }
 
       // Use a different seed for builders than for values, so they do not have
@@ -1274,12 +1258,12 @@ abstract class ValueSourceClass
 
       final autoCreatedNestedBuilder =
           field.isNestedBuilder && settings.autoCreateNestedBuilders;
-      final maybeOrNull = autoCreatedNestedBuilder ? '' : orNull;
+      final maybeOrNull = autoCreatedNestedBuilder ? '' : '?';
 
       result.writeln('$type$maybeOrNull get $name;');
       // Add `covariant` if we're implementing one or more parent builders.
       result.writeln('set $name(${interfaces.isEmpty ? '' : 'covariant '} '
-          '$type$orNull $name);');
+          '$type? $name);');
 
       result.writeln();
     }
diff --git a/built_value_generator/lib/src/value_source_class.g.dart b/built_value_generator/lib/src/value_source_class.g.dart
index 602d0ebf..b1448657 100644
--- a/built_value_generator/lib/src/value_source_class.g.dart
+++ b/built_value_generator/lib/src/value_source_class.g.dart
@@ -11,10 +11,6 @@ class _$ValueSourceClass extends ValueSourceClass {
   final InterfaceElement element;
   ParsedLibraryResult? __parsedLibrary;
   String? __name;
-  bool? __isNonNullByDefault;
-  String? __orNull;
-  String? __notNull;
-  String? __late;
   String? __implName;
   ClassElement? __builderElement;
   bool ___builderElement = false;
@@ -70,19 +66,6 @@ class _$ValueSourceClass extends ValueSourceClass {
   @override
   String get name => __name ??= super.name;
 
-  @override
-  bool get isNonNullByDefault =>
-      __isNonNullByDefault ??= super.isNonNullByDefault;
-
-  @override
-  String get orNull => __orNull ??= super.orNull;
-
-  @override
-  String get notNull => __notNull ??= super.notNull;
-
-  @override
-  String get late => __late ??= super.late;
-
   @override
   String get implName => __implName ??= super.implName;
 
diff --git a/built_value_generator/lib/src/value_source_field.dart b/built_value_generator/lib/src/value_source_field.dart
index c9580e7c..dab30aff 100644
--- a/built_value_generator/lib/src/value_source_field.dart
+++ b/built_value_generator/lib/src/value_source_field.dart
@@ -13,8 +13,8 @@ import 'package:built_collection/built_collection.dart';
 import 'package:built_value/built_value.dart';
 import 'package:built_value_generator/src/dart_types.dart';
 import 'package:built_value_generator/src/field_mixin.dart';
-import 'package:built_value_generator/src/fixes.dart';
 import 'package:built_value_generator/src/fields.dart' show collectFields;
+import 'package:built_value_generator/src/fixes.dart';
 import 'package:built_value_generator/src/metadata.dart'
     show metadataToStringValue;
 
@@ -53,12 +53,6 @@ abstract class ValueSourceField
   @memoized
   String get name => element.displayName;
 
-  @memoized
-  bool get isNonNullByDefault => element.library.isNonNullableByDefault;
-
-  @memoized
-  String get orNull => isNonNullByDefault ? '?' : '';
-
   @memoized
   String get type => DartTypes.getName(element.getter!.returnType);
 
@@ -318,23 +312,23 @@ abstract class ValueSourceField
             'The current type, "$type", is not allowed because it is mutable.'));
     }
 
-    if (isNonNullByDefault && hasNullableAnnotation) {
+    if (hasNullableAnnotation) {
       result.add(GeneratorError((b) => b
         ..message = 'Remove "@nullable" from field "$name". '
-            'In null safe code, add "?" to the field type instead.'));
+            'Add "?" to the field type instead.'));
     }
 
     if (builderFieldExists) {
       var builderElementTypeOrNull = buildElementType;
-      if (builderElementTypeIsNullable) builderElementTypeOrNull += orNull;
+      if (builderElementTypeIsNullable) builderElementTypeOrNull += '?';
       final builderType = _toBuilderType(element.type, type);
-      if (builderElementTypeOrNull != type + orNull &&
+      if (builderElementTypeOrNull != type + '?' &&
           (builderType == null ||
               (builderElementTypeOrNull != builderType &&
-                  builderElementTypeOrNull != builderType + orNull))) {
+                  builderElementTypeOrNull != builderType + '?'))) {
         result.add(GeneratorError((b) => b
           ..message = 'Make builder field $name have type: '
-              '$type$orNull (or, if applicable, builder)'));
+              '$type? (or, if applicable, builder)'));
       }
     }
 
diff --git a/built_value_generator/lib/src/value_source_field.g.dart b/built_value_generator/lib/src/value_source_field.g.dart
index 8b79f6c5..fdb2218f 100644
--- a/built_value_generator/lib/src/value_source_field.g.dart
+++ b/built_value_generator/lib/src/value_source_field.g.dart
@@ -16,8 +16,6 @@ class _$ValueSourceField extends ValueSourceField {
   @override
   final FieldElement? builderElement;
   String? __name;
-  bool? __isNonNullByDefault;
-  String? __orNull;
   String? __type;
   bool? __isFunctionType;
   String? __typeWithPrefix;
@@ -57,13 +55,6 @@ class _$ValueSourceField extends ValueSourceField {
   @override
   String get name => __name ??= super.name;
 
-  @override
-  bool get isNonNullByDefault =>
-      __isNonNullByDefault ??= super.isNonNullByDefault;
-
-  @override
-  String get orNull => __orNull ??= super.orNull;
-
   @override
   String get type => __type ??= super.type;
 
diff --git a/built_value_generator/test/built_value_generator_test.dart b/built_value_generator/test/built_value_generator_test.dart
index da15f715..a83f209f 100644
--- a/built_value_generator/test/built_value_generator_test.dart
+++ b/built_value_generator/test/built_value_generator_test.dart
@@ -223,7 +223,7 @@ abstract class Value implements Built<Value, ValueBuilder> {
   int get x;
 }'''),
             contains('1. Remove "@nullable" from field "x". '
-                'In null safe code, add "?" to the field type instead.'));
+                'Add "?" to the field type instead.'));
       });
     });
 
diff --git a/built_value_test/pubspec.yaml b/built_value_test/pubspec.yaml
index ee8ee2ef..2069693f 100644
--- a/built_value_test/pubspec.yaml
+++ b/built_value_test/pubspec.yaml
@@ -11,7 +11,7 @@ topics:
  - build-runner
 
 environment:
-  sdk: '>=2.12.0 <4.0.0'
+  sdk: '>=3.0.0 <4.0.0'
 
 dependencies:
   built_value: ^8.7.0
diff --git a/chat_example/pubspec.yaml b/chat_example/pubspec.yaml
index 06eb0562..3aba298f 100644
--- a/chat_example/pubspec.yaml
+++ b/chat_example/pubspec.yaml
@@ -6,7 +6,7 @@ description: >
 homepage: https://github.com/google/built_value.dart
 
 environment:
-  sdk: '>=2.12.0 <4.0.0'
+  sdk: '>=3.0.0 <4.0.0'
 
 dependencies:
   built_collection: ^5.0.0
diff --git a/example/lib/polymorphism.dart b/example/lib/polymorphism.dart
index 4f4cd4db..194c29f2 100644
--- a/example/lib/polymorphism.dart
+++ b/example/lib/polymorphism.dart
@@ -65,14 +65,14 @@ abstract class Fish extends Object
 }
 
 /// As with any Built Value, behaviour can be added via mixins.
-abstract class Walker {
+mixin Walker {
   int get legs;
 
   bool get canWalk => legs > 0;
 }
 
 /// As with any Built Value, behaviour can be added via mixins.
-abstract class Swimmer {
+mixin Swimmer {
   int get fins;
 
   bool get canSwim => fins > 1;
diff --git a/example/lib/polymorphism.g.dart b/example/lib/polymorphism.g.dart
index 24d36a16..c5be9a41 100644
--- a/example/lib/polymorphism.g.dart
+++ b/example/lib/polymorphism.g.dart
@@ -99,7 +99,7 @@ class _$FishSerializer implements StructuredSerializer<Fish> {
   }
 }
 
-abstract class AnimalBuilder {
+abstract mixin class AnimalBuilder {
   void replace(Animal other);
   void update(void Function(AnimalBuilder) updates);
   int? get legs;
diff --git a/example/pubspec.yaml b/example/pubspec.yaml
index ab3db7e6..f77c475c 100644
--- a/example/pubspec.yaml
+++ b/example/pubspec.yaml
@@ -5,7 +5,7 @@ description: >
 homepage: https://github.com/google/built_value.dart
 
 environment:
-  sdk: '>=2.12.0 <4.0.0'
+  sdk: '>=3.0.0 <4.0.0'
 
 dependencies:
   built_collection: ^5.0.0