When there is a tag which should be removed completely and removing the tag manually would cause bumping too many questions, the typical solution is burnination. Perhaps in some cases, the moderators can merge the tag into another tag. (If all of those questions fit into an already existing tag.)
However, some sites have a slightly different workflow for this: There is a separate tag for this purpose. Let us call it (tag-removed) - the sites which I know of use this name. When a tag is supposed to be removed a moderator simply merges the tag into (tag-removed). This removes all instances without bumping the questions.
Perhaps I should stress that (tag-removed) is different from (untagged). The latter is rather special tag which can be only added to a question by the system is some special situation, not by regular users. (The tag untagged can appear on a question as a result of migration or the automatic tag pruning of low usage tags, for example. For more details on this tag see the tag-info for untagged-questions and the links given there.)
Are both these approaches reasonable ways of removing a tag? Is there some big problem with having a separate tag for this purpose?
Could the tag (untagged) be used for this purpose instead of (tag-removed)? In the other words, are moderators allowed to merge some tag into (untagged)? (As mentioned above, regular users can't do much with the (untagged) tag but maybe moderators do.)
Of course, if the answer to the first question is that this method should not be used for removal of tags, then the second question is completely irrelevant.
As far as I can tell:
- The disadvantage of merging into (tag-removed) is that the artifact in the form of "unusual" tag remains on all those questions.
- The advantage is that it can be done by local mods and it is not needed to request help from Stack Exchange staff. (Burnination cannot be done by moderators.)
- I see also slight "psychological" disadvantage: If you need to contact Stack Exchange team before doing something, you probably think twice before doing so. So perhaps having to ask somebody to help with burnination lowers probability of hasty decisions on tag removals.
Is there something else that I am missing?
The sites with such tag that I am aware of are Theoretical Computer Science and MathOverflow. Here is a discussion on Meta MathOverflow explaining how the tag is supposed to be used. I am not entirely sure whether the tag is used as "burnination replacement" also on Theoretical Computer Science. I found only one relevant discussion on this site's meta. The usage of this tag in this way is not mentioned the explicitly, but it is mentioned that tag was created based on "similar practice and tag on MO".
From this meta discussion I can see that tag with the same name and the same purpose also existed no Mathematics, but this site no longer has such tag. (And most likely it is not going to be reintroduced there, judging by a moderators response on a related discussion.)
I do not know whether there are some other sites which have similar tag with different name. (Feel free to let me know if you know whether this is also used elsewhere in the network. My impression was that I saw some site with separate tag for off-topic questions. But I do not remember where and I am not sure whether it was used also for the purposes of tag removal.)