What do you mean it's "impossible for [you] to use this setting"? Have you tried using it with tfdata since data died, or are you just assuming that you will be on att.mvno forever because you have an old SIM card?
Yes, ST is forcing everybody over to the tfdata APN. You don't need to get a new SIM card to use that APN, either; ST can make a change on their side that causes your SIM card to suddenly start requiring tfdata and which also makes att.mvno impossible to use any longer. Typically, this happens when your monthly renewal date hits. It happened to me a week ago. Was your renewal date two days ago?