This.

Was in the same boat over a year ago when AT&T started SMS spamming about 3G sundown plans on one of my prepaid phones (Blackberry Key2) flagged as "incompatible"; to be sure, the older KEYone and Priv models are apparently whitelisted because AT&T had sold and directly supported those models at some time in the past.

Got so annoyed by the recurring, persistent warnings and no reasonable explanation why this 4G VoLTE phone would be sucking hind tit on their network in the near future that I ported the number to T-Mobile on a random Sunday afternoon (reduced monthly bill too) and never looked back.

If the UX leading up to disconnect was anything like mine, I'm honestly not sure why this event is catching so many AT&T customers off guard.