There used to be a close reason called "no longer relevant" that we were supposed to use, but it no longer exists.
What should we do instead in this day and age?
The answer is to use one of the close reasons we do have.
That close reason was removed 8 years ago according to the date of that meta question and it mentions using "too localised" instead.
I would say that our current reasons of "too broad" or "unclear what you are asking" can, depending on the situation, fit the same purpose as "no longer relevant".
The meta question you linked has an example of why "no longer relevant" was required:
The problem "solved itself", no accepted answer, nothing. It's a typical case of a "situation" which is no longer relevant.
For me "too broad" also fits, as there is no way to know what fixed it. If a problem fixes itself without any rhyme or reason then by definition the question does not contain enough detail with which to explain what fixed it.