It seems that Google has started actively blocking RCS for users who don't meet strong integrity requirements. I now receive an error stating that my device doesn't meet the security requirements for one provider on GrapheneOS. RCS does work for that provider on two other devices running the pre-installed OS.
However, there seem to be multiple issues with RCS, some of which are unrelated to GrapheneOS. For example, when trying to set up RCS for T-Mobile, I receive a different error message that simply says "failed to connect."
Perhaps GrapheneOS should file a complaint with the EU regarding Google abusing its power for anticompetitive practices, which include blocking access to a better chat experience for users who use a different operating system.
https://android.gadgethacks.com/news/when-your-custom-rom-breaks-rcs-the-hidden-diagnostic-coming-to-google-messages/
