As pointed out by Reddit user u/Ethrem, the MVNO Boom Mobile is no longer offering service over AT&T’s network. Another Reddit user, u/cllatgmail, shared part of a conversation with a Boom support agent:
The ability to renew on the yearly plan will be rescinded shortly.
At this time, I’m not sure if whatever caused Boom to pull its AT&T plans will also affect other MVNOs offering service over AT&T’s network.