RCS was meant to be a SMS replacement spec for carriers to implement but it never reached ubiquity like SMS did.

And of the carriers that rolled it out, not all of them rolled it out to the same spec either so they’re not even completely interoperable.

Then there’s the fact that many of the Google Messages features such as E2E encryption aren’t a part of the RCS Spec. They were built on top of it by Google.

And unless you’re Samsung, good luck on building a messaging app that’s interoperable with the Google version of RCS they use in messages.

In short, Google RCS runs through Google’s servers, not the carriers like it was designed for. As far as I see it, it’s just the Google version of iMessage.

arstechnica.com/…/google-enables-end-to-end-encry…

If you want to download the actual RCS universal profile spec as defined by GSMA you can find it here, missing quite a few things from the Google version you see in Messages:

www.gsma.com/futurenetworks/…/universal-profile/