Yes, iOS 18 will include RCS support
It got about three words during the WWDC 2024 keynote.
Blink and you'll miss it: Apple just confirmed that RCS support will be in iOS 18 via a three-word update at the end of Craig Federighi's update on the new software. I wish this post was longer, with more details, but they said literally nothing else about it! We'll just have to wait until the iOS 18 beta is out there, I guess.
Catch up here for all the news out of Apple's WWDC 2024.