Privacy

I've heard and read recently about a warrant that shows that the US government is monitoring push notifications on iOS and Android. This is possible because push notifications mostly work remotely, using "push notification post office" servers at Apple and Google.

I am aware that certain apps; such as Threema, Signal, Proton, Tuta, and others; use their own servers for push, but not all apps will do this. If I was to run a deGoogled Android ROM or an alternative mobile OS, how would push notifications be handled? And is there an option that doesn't involve a central server at all?