Did anyone confirm this?
Yes, I believe it's been confirmed. I reported that Signal Check Lite indicated T-Mo only for me-- the phone still sees AT&T towers, but can't connect if I try to select carrier.. Someone (sorry can't remember who) confirmed that 3UK's home site information indicates roaming on T-Mobile, but no longer mentions AT&T.
Post to give credit due: mmfacemm is the one who reported about 3UK's home site for US roaming information.
That is unfortunate, because where I am ATT signal is better than Tmobile, but who knows? Maybe, and this would be a big maybe, with Tmobile as the new signal they might be able to offer even better data price rates... otherwise this move is just to cut costs and save money vs being on ATT.
Additional question, would this be on the LTE sims too or just the Global?