No. RCS is still quite hard to actually implement. There's an open issue for it but the effort is mildly herculean.
AFAIUI though GOS does intend on providing an RCS impl eventually even if it takes years to do.
also I think RCS is way to insecure for Graphene OS - they have no way of making sure the communication is actually 100% secure.
particularly if you want to support all of the message-body features, of which there are MANY. it's very business-comms oriented.
I broadly expect third-party RCS apps to drop that though, like how essentially none support all of MMS. did you know MMS supports slideshows (I built support for this once)? 3d objects (mimetype model/gltf+json)? "timed text" (mimetype text/mp4)?
What's so difficult about implementing RCS from a technical perspective?
I'm really hoping they're doing this to pave the way for an RCS implementation. If it can be done, GOS is in the best position to make it happen, and without RCS, chatting with normies is both extremely inconvenient (large group chats straight up do not work) and categorically insecure. If I could convince all my friends and family to use Signal I would, but they just think I'm on about some weird Edward Snowden shit, and they just write me off as crazy. "Why would I checks notes download software to solve a problem when I could use the one that came with my phone?"