Which itself is not guaranteed to work even on ”stock” Android, let alone GOS — which multiple people (myself included) have been experiencing firsthand.
GrapheneOS hasn't added official support for it yet, but we're working on it. People using it on GrapheneOS have been relying on it working for many people with various tweaks but not everyone. Official support will make it reliable.
I'm also curious about the answer to this. An open-source third-party RCS implementation would be a massive boon to the ecosystem. It's obviously theoretically possible for non-Google-Messages SMS apps to support RCS (since vendors like Samsung were able to add it to theirs before deprecating it in favor of Google Messages), but it sounds like that's a non-trivial task (which is probably why said vendors ended up deprecating theirs in favor of Google Messages).
