GrapheneOS never had a stance against implementing RCS and has supported RCS at an OS level for years. The issue was that the only available RCS app in practice is Google Messages and it requires privileged access for Google Play services, which goes against the sandboxed Google Play approach. We worked around it by making it so that the access granted to Google Messages when it's set as the SMS/MMS/RCS app also applies to Google Play services where part of the implementation is done.

iOS does not currently implement end-to-end encryption for RCS. End-to-end encryption for RCS is exclusive to conversations between Google Messages users. Apple has said they'll implement the new MLS end-to-end encryption for RCS but has not done it and has provided no timeline for doing it. It took them a very long time to implement basic RCS support and this will likely take a long time too. Google Messages has not yet moved to the new MLS encryption, but it will need to do that too in order for iOS implementing it to provide end-to-end encryption across them.