It’s kind of open. It’s pretty much open for carriers to implement on the server side, and for OEMs to develop on the client side. There is an open source client in AOSP’s RCS Test App, but for one reason or another, as far as I know nobody’s attempted to implement it in an actual usable client app. I don’t believe there’s a server reference implementation. And, in the US, all the carriers’ RCS services are run exclusively by Google, so there’s no real point in attempting to set up your own server. Apple might be able to navigate the politics with carriers and with Google to make something work, if it wants to, but it’s really not a standard for us to play with.

Use Matrix Instead.