It was Versus mode against an arbitrary opponent. I'm not sure that you can choose the format of the game. You can filter your decks by type (Theme, Standard, Expanded, Legacy) and the corresponding decks will show up but when you hit Play, I think you can be matched up against other types (including I guess Legacy where these banned cards are legal). Unless it's only when they can't by find an opponent with a matching type for you that this happens?
Ah, OK. I figured Legacy offered a greater choice of cards and it might show up there, but not even Legacy legal, got it. I guess you'd need to be able to choose Unlimited in the dropdown list, but it's not there. Perhaps it used to be there and the people playing Reset Stamp and Red Card have an older version of the software.