@fri @Hirom HeliBoard is FOSS, on F-Droid, does predictions, works offline, and is actively maintained.

If you want swiping, you'll have to install a proprietary Google binary though. Basically download https://github.com/erkserkserks/openboard/tree/46fdf2b550035ca69299ce312fa158e7ade36967/app/src/main/jniLibs/arm64-v8a (probably this version, although there are others for other less common processors) and add it in the HeliBoard settings.

Homepage is @
https://github.com/Helium314/HeliBoard (and it links to the page that contains that binary library, including other architecture versions).