On Super User, we have quite a few tags that shouldn't exist. Unfortunately, some of those have a lot of questions, and despite our clear directives in the tag wiki excerpt, the tags continue to be applied to new questions.
I propose a very mild and hopefully fairly easy to implement alternative. Moderators should be able to mark a tag as obsolete, which would visually restyle the tag in the ask-question page and the tag's info page, but not block it entirely (it would be bad if an accident froze the site's biggest tag). This strategy minimizes the impact of mod mistakes (even if very rare) and keeps them easily reversible.
Then, a column would be added to SEDE to indicate a tag's depreciation, in the Tags table. I include this point because Burgi had the really clever idea of using SEDE to find First Posts review items with bad tags, and my
LIKE-based logic isn't very good at identifying the problematic tags from their excerpts. Such a column could help with other bad-tag-related science that people might come up with. It and the visual restyling would also be a good way for users interested in tag cleanups to know that a tag is officially slated for removal (without needing to search through meta).