It worked for me on GrapheneOS for quite a while, but a couple months ago things started breaking and I no longer have it enabled. There's an absolute behemoth of a thread discussing the issue, and unfortunately it's still active which I assume means I'm not safe to enable it again yet. If you want some light reading to help put yourself to sleep: https://discuss.grapheneos.org/d/1353-using-rcs-with-google-...