@MartinSleziak Indeed this is a great curiosity for me... ultrafilters are very classical, and among the basics of most people interested in set theory, logic, and general topology, also used by some people in Banach spaces etc... "ultrafilter" appears in more than 300 questions, it's even a tag, for which there are more 160 questions. I'd thought the question would be closed within a few hours.

Possibly people are more reluctant to close a question when asked by a user with high rep, but this is not the only reason here; notably the number of upvotes rather shows that a very well-known classical object can also be very well-unknown.

The fact that a question by high rep user is handled differently than the same question by a new user is not really surprising. (In fact, I think I saw some people on meta arguing that this is the way it should be.)

And, of course, it seems common that MO does not really follow it's own rules. Sometimes a question which is closer to freshmen exercises that to a research level stays on the site and is well received.

It seems that Todd Trimble's description of what survives on MO seems to be spot on - I'd say that it's more about whether the question seems interesting to MO users than whether it fits the type of question described in the help center as "on topic" for this site.

And also MO users are more reluctant to do some maintenance or community moderation than users on Mathematics, for example. At least, that's my impression as an outsider.

I don't think that much can be deduced from the number of upvotes. It is often higher on the HNQs. (And the question has a "clickbaity" non-descriptive title, which might attract users from other sites.)

Anyway, I wanted just to point out a question which is both HNQ and has close votes; which is a bit unusula.

But it happens, as I pointed out also in this question on meta: Should moderators in some cases remove a question from the network-wide hot questions list?

The SEDE queries mentioned there return 32 questions which were in the HNQ and are closed and 38 HNQ questions which were closed at some point.