> It's just a JAR (no native components)

I’ve reversed it, and rebuilt an alternative

The jar you include actually opens just an IPC channel to the Google Play Services framework, which runs with system permissions, and handles the actual stuff.

You can’t implement your own FCM without having root access on EVERY Android phone out there.

Maybe not. Although there's this guy: https://eladnava.com/pushy-a-new-alternative-to-google-cloud..., it seems like his library doesn't work very well after Nougat.

From Android 6 on, Doze just terminates apps running in background after a while. The only network access that is allowed is GCM.

Which causes the entire issue.

I wrote a complaint to the EU privacy official responsible for them and the EU antitrust committee.

That's an excellent idea! Do you have a blog or something where you're going to post when (if) you get a response?