Five Figure Fanwork Exchange

generally "exists in this fandom" is a sensible requirement for characters in tag sets.

Agreed in general. This kind of Big Two official AU isn't exactly an independent fandom, however -- it exists entirely in tension with "the canon" in the way an AU fanfic does. If the AU doesn't specify that its leads are the only primary continuity characters present (this does sometimes happen), it's assumed by fandom and generally also, explicitly, by the creators that the full cast exists in the AU and most of them just haven't passed through the eye of the "camera". These characters aren't considered to have been borrowed in; their native existence is implied as part of the premise of the AU. "What would so-and-so be like and how would their origin story play out in this AU" is a really common exercise in this context.