I've used Unexpected Keyboard and FlorisBoard and I'd say that they're great introductory FOSS apps for most users (users inexperienced to sudden changes)
Unexpected Keyboard uses Nix in their codebase :D
Thumb-Key is maintained by dessalines whom is a Lemmy dev
Florisboard is starting to convert their codebase to Rust,
ah unfortunately I had to recently take down a post here that recommended HeliBoard, - as that project is in violation of OpenBoard's GPL-3.0 license
For some unknown reason the developers of HeliBoard decided to multi-license with Apache-2.0 and CC-BY-SA-4.0 - as I wasn't able to find any information that they received explicit consent from all members of OpenBoard about relicensing/multi-licensing
Hence I can't recommend anyone using that keyboard app until further information is disclosed - afaik multi-licensing GPL-3.0 is only possible with other specific licenses such as AGPL-3.0
just to double-check I searched online and found this: license-compatibility > Likewise, CC BY-SA 4.0 explicitly permits relicensing modified versions to GNU GPL version 3, but GPL version 3 does not permit relicensing to CC BY-SA. This issue should never arise for software code; Creative Commons says its licenses are not meant for code, and says that the license to use for code is the GNU GPL. But there are other kinds of works, such as hardware designs or game art, where you might have occasion to merge material released under CC-BY-SA with material released under the GNU GPL. This can be done through CC BY-SA's explicit relicensing permission.
> Unfortunately, CC BY-SA 4.0 does not permit relicensing to future GPL versions. What you should do, when you relicense material under CC BY-SA 4.0 to the GPL, is specify yourself as a license version proxy to indicate whether future GPL versions have been authorized for that material. If someday there is a GPL version 4 and Creative Commons decides to allow relicensing from CC BY-SA to GPL version 4, you as proxy will be able to retroactively authorize use of that relicensed material under GPL version 4. (Alternatively, you can ask the authors of that material to give permission right away.)