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Google is silently blocking RCS messages on rooted Android phones and custom ROMs
(www.androidauthority.com)
This is a most excellent place for technology news and articles.
Verge article where the claim is due to ad blocker mitigation and not browser specific.
You said that you heard it was ad blocker related too. So the initial wave of reports about it being Firefox was inaccurate. Every article about it all sourced a singular reddit post. Just some social media "journalism".
Anecdotally, which I understand is not a great measure, I didn't experience this when it was first being reported on. But I pay for YouTube premium, so maybe that's a difference too