It technically isn’t proprietary. But many implementations are reliant on Google’s Jibe system. So even if you’ve avoided Google completely. If you use RCS there is a strong chance all your messages are going through Google.
RCS relies on the carrier to implement. With many carriers using Jibe, even if your doesn’t the people you message likely are. So you can’t get away from Google.
At least with iMessage, WhatsApp, Telegram and Signal (plus Allo, Hangouts GChat, meet, GmailTalk etc). You know who controls the messaging service. You can then made a decision to engage with that messaging service.
With RCS this isn’t clear. You may think your using your carrier or the person’s your communicating with carrier. Or you may be using Google’s Jibe. Or some other implementation.