https://github.com/Hirohumi/rust-rcs-client

Someone has written an open source RCS client prototype, but it has been only tested in China, where carriers do provide their own RCS servers as they are supposed. The author has not tested it with Google’s servers, which are probably blocked in China.

If you want to use SIM card based authentication, you need to have the app installed as a system app. That however is not an option for Google’s servers anyway, since they need to be able to work without carrier co-operation. Google uses SMS based authentication instead.

There does not necessarily need to be anything in Google’s servers that would reject non-Google RCS implementations: the SMS based authentication is defined in the spec, too.

Personally, I would not want the Google’s proprietary implementation to serve an API, but there to be a fully open source client instead.