Looks like:
1) Too few developers to keep up with new Android releases, or with devices becoming obsolete over time. They got stalled trying to update to Android 11 (2020) and never finished.
2) Refusing to support newer devices that doesn't meet their demands (e.g. must have a removable battery). Turns out, the market doesn't respond to the demands of a small open-source project.
I don't understand the need to be all the time updated with the bleeding edge upstream when there aren't enough devs/contributors. Every underfunded understaffed project fails the same way: tying to keep up.
Just skip a version. The world won't end.
They already did that. The previous release was based on Android 6, and is functionally useless nowadays given that it only runs on a couple of Samsung devices all released around 2011-12.
If you're trying to maintain a fork of a large open-source project, and especially one which needs to interoperate with external systems like Android, keeping up with upstream releases is not optional. The upstream will move on without you.