EDIT: This was posted as a response to the original version of the question which explicitly mentioned also tag edits.
I will try to address mainly the general question about editing closed questions - as far as I can tell your question is intended to be general and not only about the edit you linked to. (We can leave comments and chat for discussion of this specific edit. But certainly the reason that I was the editor in question was a contributing factor why I decided to post an answer. So I briefly touch also this specific edit in this answer - but that is not the main point of the answer.)
I think that allowing only the OP to edit closed post is not ideal. (On the other hand, in the first days after putting on hold it is definitely preferable if edits comes from the OP at least for salvageable questions, since the first edit - if done within five days - pushes the question into the reopen review queue. But that's a separate topic.)
There might be also some other arguments against the suggestion "only the OP can edit a closed question", but in fact you pointed out one important one already in your question - namely the tags. If only the OP would be allowed to edit closed question, it might be impossible to remove tags that exists on such questions. To list some recent examples, we might still have the misspelled tag probabilty. Or even if the community agreed that specific tag for ncatlab is not suitable, a single user would be able to keep the tag alive simply by refusing to remove it from a single closed question. More importantly, closed questions often come from inexperienced users who are not familiar with tagging systems. And in case of older questions it might be possible that the OP no longer visits the site, so we cannot really expect edit to come from the OP. (Of course, the tag can be removed also when a question is deleted, but it seems that MO users do not vote to delete questions very often.)
You have also specifically mentioned old questions. It is true that old questions should not be bumped often, but with moderation it is acceptable: Do we have an unofficial quota on how many old questions one should bump for minor edits in a single day?
You have also asked this:
I am not sure if that adds anything to the question. ... I am not able to see the point of editing a 6 years old question.
I think having questions correctly tagged is useful. It helps when searching and also when choosing the questions in the related questions list in the sidebar. In this specific case the question is closed but it seems unlikely to me that it is going to be deleted. So if it stays, it is probably better for the site that it is correctly tagged even if it is closed. (But I definitely admit that this is more important for questions that are not closed. And it is much better to make such edits when the question is new or if it has already been bumped for some other reason. Probably I would not have made this specific edit if it wasn't a post with a deprecated tag.)
Some benefits of bumping this question:
- The question gets to the front page and users who see it might reconsider whether it is worth keeping (if not, they might vote to delete it) and also reconsider whether it is worth reopening (although in this specific case this is unlikely scenario).
- Whoever is curious why the question was bumped and looks at the edit summary might be reminded that there are deprecated tags. And perhaps the reminder helps that the user helps with the removal of such tags when they see them.
And a possible downside is only that an old question will be for a short period on the frontpage - which is not that bad. (Considering that I have over 100 helpful spam flags - which means that I contributed to remove quite a few unsuitable posts from frontpage - I definitely feel less guilty if my action leads to some post appearing on the frontpage, even if the post is less than optimal. And I am quite careful not to bump more than one old question per day - which is way below the suggested quota of three questions per day.)