cb474
Scroll back a bit in the history of this chat. There's an eztensive discussion with cited sources from just a couple months ago about this.
TL;DR
RCS is an open carrier standard, but you have to be an FCC Certified Carrier to run the server. Google used to run them all, but as of Sept 19 this year, all Carriers now have to run their own, though they're still pretty much all based Google's server code.
There's a big gap between an API and a compatible client, especially when there's only really a single server implementation. No one has successfully developed an RCS client independently, Apple got Google's help for their private implementation, otherwise there's only Google Messages'. The server implementations all have semi-proprietary carrier authentication as well, so it's nearly impossible to test am open source implementation against a real server.
On top of this, all carriers seem to now require IMS_SINGLE_REGISTRATION, which is a new Android permission baked into ASOP that is only available for System apps. This is what GOS has been trying to find a solution for since the prior backward compatibility alternatives are no longer supported.