Is it best to remove tags of closed questions? Or is it best to update their tags to keep them current, just like any other question?
I think editing the tags of a closed question can make sense in various cases, for example:
We want to get rid of a tag that is (now) only used on closed questions.
Remove it, otherwise this tag will keep existing (until the closed questions get deleted, if at all).
No category tag.
Add it, otherwise it’s likely that many domain experts never see this question, and they are the most suitable users for deciding what to do with the question (if it can be rescued, if it’s a duplicate etc.).
Wrong category tag.
Remove it (and add the correct ones, see "No category tag" above), otherwise domain experts will keep finding unrelated (closed) questions when browsing their tags.
(And if editing a closed question anyway for other reasons, it should be retagged, too, of course.)
Examples for a closed question that …
asks for a Firefox add-on but misses the firefox tag: add it
asks for a free Firefox add-on but misses the gratis tag: don’t add it
asks for an extension for any web browser but uses the firefox tag: remove it
asks for a text editor but misses the text-editors tag: add it
asks for a Linux text editor but misses the linux tag: don’t add it
asks for a desktop text editor for any operating system but uses the linux tag: don’t remove it
(Why removing firefox but not linux if it’s not a requirement tag? Because, I think, "Firefox" is more like a category, with likely more domain experts, while "Linux" is way more broad and it’s rather unlikely that there are domain experts for all Linux software in general. An exception would be if the closed question already has good answers, in which case removing linux would be appropriate.)
A good question I'm also thinking about from time to time: though "closed", those questions stay and are often kept as references – especially with "duplicates", which serve as pointers to the "originals"; which allows for finding those with different search patterns.
Still, I'd say: Unless worth re-opening for some reason, those should not be edited at all but kept as-is (if not even deleted at some point), for two reasons:
- editing them pushes them into the review queue: to get your edit approved (unless you have enough rep), and for whether the question should be re-opened
- editing them might bring them back to the "front page" as question with new activity
So unless there's a very good reason, I'd say refrain from editing closed questions. Just adding/removing/replacing a tag IMHO is not enough reason for that, usually ;)
Expanding a bit on Izzy's answer. Closed questions are of several types:
Questions closed as duplicates: These questions remain as a reference and serve as an additional link to direct search results to the answers of the original (sometimes even less popular) question. In this case, I think that the tags on such questions should be already pretty close to the original one, and ideally no editing is needed, unless it's to match the original, or a part of a tag cleanup
Questions closed for reasons other than being a duplicate: These questions are in a sort of a limbo, and I think that whether or not their tags should be edited/are worth editing depends on the fate of this question.
- Questions that are going to be deleted (See the conditions here) should not be edited unless you believe that your edit could save the question. In that case, you should also edit the title and/or body, and nominate it for reopening.
- Questions that are staying, like upvoted ones or ones with accepted answers but are closed as too broad: I'm not sure what's the desired outcome from editing those. If the question can't be answered objectively, then it doesn't make much sense to help draw more attention to it.
Conclusion: I see very little reasoning behind minor edits on closed questions. I think they should only be edited when that can lead to their reopening, which usually means more than just editing the tags. An exception might be cleaning up tags deemed obsolete