Yes, Google only made it available for Google Messages. They don't have an SDK or API you can use to make your own clients or servers. Google also didn't put it up for standards with the GSMA or any other standards body, at least not publicly. There are no records of it.
There are some older submissions here you can probably find using one of the HN search sites about this, but IIRC those didn't really have any internal Google policy about this, they kept it all pretty private. The only 'leak' I remember about this was the thing where manufacturers that preload Google Android have to ship Google Messages to get RCS support from Google, otherwise they can't have it. Also means you can't have RCS without Play Services.
There are some older submissions here you can probably find using one of the HN search sites about this, but IIRC those didn't really have any internal Google policy about this, they kept it all pretty private. The only 'leak' I remember about this was the thing where manufacturers that preload Google Android have to ship Google Messages to get RCS support from Google, otherwise they can't have it. Also means you can't have RCS without Play Services.