Here, I just upgraded to iOS 18 before lunch because of RCS and a friends abroad in another country who’s on Android to message him.
My watch, iPad and Mac tell me it’s sent as SMS but the iPhone lastly sends it as RCS. So it’s great 🙂
Also to mention, he has only an iPad with iMessage. I had fears that it would always try to switch to iMessage with the email, but it keeps using the RCS as (at least for me) preferred.