"Smart Features" in the Messages app have always been explicitly processed only on-device. This is a big change if it is different than that.
I'm betting they'll make this opt-out, which is fucking shady as hell. And worse, I bet opting out your own messages doesn't stop someone else that is opted in from unknowingly/unintentionally transmitting all your messages that they received. Ugh.
RCS stands for Rich Communications Suite. Its a standard developed by google to make texting more feature rich like dedicated messaging apps like WhatsApp or Telegram. Pretty much any stock android app uses it these days. If you want to know more here's a pretty good explanation of it. https://www.androidcentral.com/what-rcs-and-why-it-important-android
End to end encryption if both parties are using google's implementation, delivered/read indicators, far better data limits for media, works without phone signal over data, message limits increased, free and international, fallback to sms if recipient doesn't support it
I bet opting out your own messages doesn't stop someone else that is opted in from unknowingly/unintentionally transmitting all your messages
On a somewhat related note, this is my main gripe with Beeper, or Matrix bridges in general. Someone could be using it, giving encryption keys to a third party, letting them handle your chats with them, and all of this without you ever even knowing.