Google Messages will support texting 911 via RCS starting this winter, offering features like location sharing and read receipts. This upgrade improves emergency texting which is already supported by over half of US dispatch centers. Google collaborates with RapidSOS for enhanced responder info. This announcement precedes Apple's expected RCS support in iOS 18, aiming to broaden RCS adoption.
It's "open" except for the proprietary stuff Google layered on top, and that the only RCS implementation Google has allowed on Android is their own, and a couple of derivatives of their own, where they had to sign an agreement with Google.
So in actual practice, not open. Even if the standard technically is.
Right now, if you want to make an RCS app independent of Google, you'd also need to make a new OS. Or fork Android and do major work on it.
Google owns Rcs. Nobody else is capable of doing what they're doing with it, and they don't let anyone else play.
No third party apps, no way to choose other implementations.
Nobody really gives a fuck about the standard itself being "open" if there's no ability for an end user to have any choice is the matter. It's Google or GTFO.
But aside from the two of them, yeah, closed. Basically the protocol the servers use to talk to each other is open, but whether they'll want to talk to your server is undocumented and unlikely, and the protocols Google and Samsung use to talk from their servers to their respective apps are closed.