KeyGuard is proprietary on Android. Certain features are locked behind a subscription or one time purchase.
I'm starting to also hate on this model. People who claim to be open source but still throw hurdles and obstacles at anyone trying to either use or contribute to the project. I understand people need to support themselves financially, but I have donated to many free projects and open projects in my life. Whenever I see a paywall it makes me not want to use the project, however interesting it might be. It sucks they are usually the only almost decent android clients. (Looking at Symphonium and their developer's shit ass attitude too)
EDIT: Just tried it and it seems like vaultwarden login and custom certificates on linux have been fried since December. It is currently a dead-end for selfhosters.