I just got a new phone, and realized that the only game I occasionally play, MathDoku, won't run anymore!
This was unacceptable, so I wrote my own: https://magnusm.uber.space/mathdoku/
Github: https://github.com/magnusmanske/mathdoku (GPL, feel free to run it on your own server if you prefer)
This is a "progressive webapp", you can play it in the browser, or install it like a "proper" app on your mobile (Chrome/Android trhee-button-menu, "Install app"; other borwsers/platforms, please let me know!). It will keep your game stored even during reboots.