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Google Messages just flicked the switch on end-to-end encryption for all RCS and group chats
(chromeunboxed.com)
This is a most excellent place for technology news and articles.
Google RCS and RCS the standard are not the same thing. Third party apps are not allowed to plug into Google RCS. This is why you won't find any RCS open source alternative for Google Messages.
Google is just trying to promote their own walled garden under the guise of an open standard. If they were genuine about this they should allow you to just use any replacement, just like you can replace the stock SMS app on Android.