Signal always contains multiple proprietary libraries.
The only difference with the Web version vs the Play Store version is that it has its own app updater.
Signal uses the following proprietary libraries:
- firebase-messaging for push notifications
- play-services-maps for maps and sharing location
- play-services-auth for performing reCaptcha checks during signup
- firebase-ml-vision for face detection for the ‘Scribbles’ feature (?)
Please consider using XMPP/Conversations/Dino/Gajim, which can be a fully FOSS stack.
It doesn’t provide as much metadata protection, but it is trivial to run your own server (ejabberd/prosody) for yourself, your family, and your friends.