EditPlayerNBT

A mod that just removes the line preventing player NBT from being modified

NBT values that change player state instantly

• abilities.flySpeed - alters vertical flight speed
• abilities.walkSpeed - alters walking speed
• abilities.invulnerable - makes you invulnerable
• foodLevel - hunger
• Fire - time on fire
• Inventory
• EnderItems - enderchest items
• Pos - position
• AbsorptionAmount - Amount of absorption

Nbt values that change player state on relog

• Rotation
• SelectedItemSlot - slot selected
• XpP - individual xp points
• XpLevel - xp levels
• XpTotal - total xp

NBT values that don't change state when edited

• Motion - velocity
• abilities.mayfly (resets on reconnect) - whether you're able to fly
• abilities.flying (resets on reconnect) - whether you're flying
• Dimension (resets instantly)
• playerGameType (resets instantly) - gamemode

There are more that I haven't checked yet