The RCS protocol is universal. Carrier RCS support is minimal, though, and third-party RCS support was never part of the spec and essentially unimplemented.

Google had to pretend to be everyone's carrier to make RCS work because the GSMA spec assumed everyone would download/install their carriers' messenger apps to use RCS, like you would back in the day with SMS/MMS. This expectation was broken the day Google allowed app developers to write third-party SMS apps, but that hasn't bothered the spec people so far.