I'm wondering if we should have a tag for the set theory NFU. On the one hand, I personally would like to see the tag stick around, since I think NFU is a very interesting and potentially useful set theory. On the other hand, as far as I can tell, only three questions have ever been asked about NFU (counting one I asked today).
I don't think three questions is worth having a tag for. I even doubt a
new-foundations tag would be viable.
If we are to do something in this area. I would prefer to have an
axiomatic-set-theory which wouldn't be as likely to attract elementary questions from askers who don't really care about axiom systems as the current
set-theory. (They are supposed to go in
elementary-set-theory, even relatively advanced ones about ordinals -- but elementary-foo tags are a blight in general; they are hard to discover for the users who need them most).
Tags are fairly cheap, but not free -- at a minimum unused tags have the downside of showing up when the system suggests tags for new question, creating confusion for new users who have to guess at which tags are actually appropriate for their new questions.