Yes, microG exists and is amazing — but even it is only replacing an absolutely tiny part of the APIs, and even those it can’t replace fully yet and a lot of work is still to be done.

I remember days when Android was still open, nowadays I’m fighting against the ever-increasing restrictions with my open source apps with every single OS update.

No, no, no - GP isn't saying replace the bits that support the proprietary Google services.

They're saying that there's easy and simple drop in replacements, app and service wise that can hook into better services/ones you host.

There are? Now that’s interesting, because I’ve spent the past 5 years developing open source Android apps and such projects. (And e.g. my app Quasseldroid is also on F-Droid)

I’d be interested in what existing, working solution for a FCM replacement there is? Replacing the apps is simple, replacing the services is the hard part.

If there is a working open FCM alternative, please link it, I’d immediately add support for it in my apps.

If you're reading this, and would like to see a solid selection of these: www.F-Droid.org