Sharing my data point:
I got a new pixel 9a a week ago and installed GrapheneOS, first time user. Mint Mobile.
I migrated from a fairphone 4, running the fairphone OS for that phone which was using RCS no problem. I assumed the RCS issues were due to my moving phones rather than anything to do with GrapheneOS, so I went down that troubleshooting path. I tried everything you can think of, as far as disabling RCS via Google's servers, etc.
Today, I tried everything I could think of or find online as far as enabling permissions, compatibility, delisting with my service provider, etc.
No matter what I do, messages will always list RCS status as "Setting up..."
If I take out my Sim card and go back to my fairphone RCS works instantly and without problems, so at least on my end it seems like the issue isn't 3rd party android versions, as much as something with GrapheneOS, but I can't figure out what since I have tried almost every compatibility or permission I can think of.
Bad timing on my part haha, but I think I would rather use GrapheneOS without RCS than the other way around still.
A deeply cynical (and almost certainly wrong) part of me wonders if google did something that screws up RCS for GrapheneOS right as Linus for LTT is wrapping up his trial/review for GrapheneOS....
(Sorry if I left any typos, new keyboard)