I recently had the problem that none of the existing self-hosted todo apps was simple enough for my purpose, so I thought I'd develop my own (pure todo).
https://github.com/sandreas/pure-todo
Despite having yet another todo app with the next best JavaScript framework, I decided to go for a learning project / feasibility study to use as little dependencies as possible. That basically means: Vanilla PHP, Vanilla JS and Vanilla CSS (except the qrcode lib and the icon fonts).
So here it is, lots of rough edges and missing or unfinished features, but usable and at least I learned something :-)
Maybe someone finds the time to provide some feedback.
Thanks.