https://store.kde.org/p/1152230/
Redesigned GUI for the kdeplasma-addons notes widget, where it's organized as a list using GUI components.
Depends on the plasma-widgets-addons package (sudo apt install plasma-widgets-addons) which used to be called kdeplasma-addons before it was broken up into 3-4 smaller packages in Ubuntu.
Some awesome users seemed to have packaged this applet under plasma5-applets-todolist.