RCS and its support in the Android Messages app is conspicuously absent from mention. I wonder if there are any RCS supporting open source messaging apps that don't require giving your message contents to Google. Or is RCM somehow closed off from third party apps?

Edit: seems that according to https://www.netmanias.com/en/post/techdocs/8154/lte-rcs/gsma... it's entangled to IMS which is to my understanding walled off from normal apps. You can't, I think, make VoLTE calls through your carriers IMS SIP infra with Android SIP apps or SIP implementing dialer apps from app store etc. I hope I'm wrong.

The misery of implementing RCS is what pushed the Matrix team into creating Matrix.

We’re now also working on the identity interop points that ekr highlights.

Interesting! Is there anything public written up about this origin story?

A few months ago I had thought about this topic, what I had in mind was some sort of protocol for Messaging which Messaging Apps could agree to adhere, thus making interoperability possible.

As the article says, that’s not a trivial problem, nor do these apps have the incentive to adhere to such solution unless they are a new player.

For the protocol Matrix would be a good fit but all the other points you mention are valid. Its not trivial and there is no incentive.