From GrapheneOS:
Google and carriers deployed changes to RCS which have been breaking it for many users on the stock OS including with certain carriers and in whole countries. The changes appear to have specifically impacted GrapheneOS users too. It’s not related to our September 8th update. We’re working on it. You can find many recent threads about people having issues with RCS in Google Messages while using a stock OS with Google Mobile Services. There are articles about how some carriers and countries no longer have it. It still works for most people but the changes definitely regressed compatibility. GrapheneOS users were impacted by these recent changes much more than other users. We don’t know why yet but we’re working on determining that and restoring compatibility with Google Messages for people who use it. We’ll try to get compatibility with the new way it functions implemented very soon.
It looks more grim on the GrapheneOS forum where the official account responded to someone:
…This was not caused by a GrapheneOS update but rather the way RCS is provided for Google Messages changing significantly to a way which we haven’t added support for using yet. We aren’t getting help with development so expect it to take a while to become supported.
Here’s an article talking about RCS being down in some countries for over a month (even on normal android devices).
Unlike previous times when Google Messages RCS stopped functioning due to app compatibility (starting the infamous RCS thread on the GrapheneOS forum), this change appears to be server-side and intentional.
In some countries so far, Google appears to have turned off its Jibe servers and returned RCS control to carriers. With many international carriers still not supporting it themselves (and with little incentive to do in places where other apps like WhatsApp are already popular), this appears to be a potentially permanent death to RCS.
However, it’s still uncertain if this is related to RCS no longer working on GrapheneOS devices, since even GrapheneOS users on carriers that have historically supported RCS are experiencing issues.
In the worst timing possible (unless this is directly related 
), this potentially permanent breakage comes just in time for iOS 26 that is being released this month, which shows signs of supporting end-to-end encryption for RCS messages between iOS and Android users.