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I've often come across very popular questions (usually on Stack Overflow) whose answers date back as far as 2008, even though the question is still relevant today as 2019 approaches.
It has often frustrated me that there isn't a way to re-open a question for updated answers short of placing a bounty on it, which I feel isn't really the right user action for the situation in question.
I don't think users should have to use up their own reputation just to keep site content updated and relevant. On some level that effectively disincentives people from improving site quality. It also means users without a lot of reputation points can't do it.
Similarly, "flagging" the question doesn't really fit either, at least with the current flagging options: it's not a problem question (neither spam nor abuse) and it doesn't need moderator intervention.
It would be nice to be able to nominate a question for updated answers. I think all users should be able to do this, although of course there would need to be some sort of constraint so that every person stopping by doesn't demand an update to a 6-month old question just because they aren't happy with current answers.
I would personally suggest the following properties and constraints as a starting point:
only possible on questions that are more than two years old. Ideally this value would vary depending on the age of the subject matter in question, but that's probably not realistic to do on a tag-by-tag basis.
any user could nominate the question for updating, and it would be placed into the Review Queue for other users to verify before approving.
users with sufficient number of reputation points should be able to by-pass the review queue for this.
once approved, the question should be essentially re-opened and appear in the question queue as normal.
the authors of previous answers to the question perhaps should receive a notification informing them of the event and inviting them to update their answers.
the author of the question should be invited to select a new answer after a suitable period of time has passed.
existing answers should be initially marked with a banner noting that they may be out-of-date, to draw users' attention to the fact that the number of upvotes may no longer accurately imply quality or relevance. This would go away if the answer were edited, although on some level I have to question the idea of an answer keeping its upvotes when its content has changed.
But that's how the site has always worked, so perhaps there could be some sort of versioning system for answers, some way of indicating that X% of an answer's upvotes preceded its major content update.