Lichess is also open source. They are transparent with their finances and have taken on a developer who is upgrading their app(s). I say plural as they are keeping the old one with minor fixes but building a new one from the ground up .

The new app can also let you play puzzles while offline and well as play a bot offline.

Major variants are supported like 960, triple chess, atomic, and antichess.

And I think they’re also big on not having any trackers on the site too.