But if you don’t have firebase, Signal uses a websocket. It doesn’t have a hard dependency on proprietary software (neither directly nor indirectly - through the open source firebase that depends on proprietary software) for notifications, so it’s fine as long as the library that it uses is open source imo
You suggested using a MicroG library, but if it also depends on GMS, then it won’t be any different. Not sure if it does depend on it though (or can use unified push/a websocket if it’s not available)