Please explain how a question that is off-topic is a good fit as a closed question for the site, but a bad fit as an open question.
I don't know enough about security to know exactly how that question fits or does not fit. What people tells me implies it doesn't, but I'll give advice on the general topic of closed questions getting deleted. This will hopefully make the specific case here be crystal clear.
With the exception of duplicates (which we keep around for searchability), closing is intended to be a temporary state for a question. There are only two states in the future of a closed question - getting deleted or getting reopened. The primary purpose of closing is to serve as a sentence to eventual deletion. However, it is not the end of the world, as the closure may be identified as erroneous or the question may be fixed up, resulting in its reopening.
The end takeaway here is that unless a question has some chance to be considered for reopening, it should be deleted. Sometimes a question is borderline, or is the subject of an extended discussio of the site's scope. This is about the only time that a question should remain closed for extended periods of time. If the question is definitively out of the site's scope, and there is no business to be had even spending any time arguing to reopen the question, then it is better off to delete the question. Otherwise, you are leaving an avenue to reopen elements that you do not believe should ever be reopened.
It is often spoken of that a closed off-topic question could be left on the site "as an example of what doesn't belong". This sounds great on paper, but in practice it doesn't work out. Recall the earlier point about duplicates - closed questions are still entrances to the site. Keeping junk you do not want means that people who are looking for said junk will end up finding your site. Most of the time, you do not want that.
The other salvaging point one might look at is the answers on the question. An excellent answer may protect a pretty horrible question, that's after all why we have the Reversal badge. However, ask yourself when looking at a closed question with a great answer, "does the answer salvage the question to the point it shouldn't be closed?" It is unfortunate that a great answer may be lost due to being in the wrong place, but you shouldn't spare the world's greatest turkey recipe just because someone asked how to keep his dinner safe from ravenous relatives. Your business as a site is to host information related to the scope you set - the great and wonderful content on your site should reflect your scope and not what is outside of it.
The fact it hasn't been deleted or migrated away doesn't mean it is a good fit. Some other options: