Well signal is full of proprietary software even though its core is Foss. That’s why molly doesn’t support absolutely proprietary operating systems
The one anti-spam module in the server code and… what else is proprietary?
FCM? The thing they need to give android users (with Google play services installed) notifications?
They have a websocket built in. So they dont really need FCM even on phone with GPlay.
The websocket is fairly energy efficient, and much more reliable than FCM, in my experience.
Signal on the Google play store isn’t FOSS but it facilitates the delivery of notifications to the user. The protocol itself is open source and notifications are handled securely even with Google play services on the phone. If you want a FOSS version, use the APK on their website, it has an auto updater.
I use molly because it has a F-droid repo and is more trustworthy in my mind