First off, if it meets close criteria vote to close it. This particular question, while it meets several criteria, probably fits best as "Primarily Opinion Based", which it has since been closed as. There really isn't any question about whether or not to close it. Historical locks come into play more to deal with:
People constantly trying to reopen offtopic questions, just because they're that popular
The question being deleted, even though it has really valuable content.
For the case of #1, you'd only need to flag it requesting a historical lock if you see the question being reopened/closed over and over again, which is not the case here.
For #2 you'd only need to flag requesting a historical lock if the question is deleted, or looks like it might be deleted soon (i.e. it's collecting delete votes).
I don't see this question as being "so absolutely amazing that we must keep this around because the world is a dramatically worse place without it, so even if the question was facing deletion I don't see it as needing a historical lock to prevent it. There are only a small handful of historical locks. They aren't for, "pretty good, somewhat useful, or kinda okay" questions. They're for, "WE MUST HAVE THIS!" questions.
Having said all of that, if you do feel a question needs a historical lock, there is no specific flag for it; use a custom flag saying that you feel a historical lock is warranted, and explain why. Alternatively, because historical locks should be for questions of enormous value, you can potentially create a discussion on meta asking if you feel a historical lock is warranted. This should be done especially if there is controversy over whether or not to lock it. This will allow an in-depth discussion to take place, with a mod to act on whatever the community consensus seems to be.