Sometimes we decide a tag that is in use is not a very good idea and we remove it, perhaps by editing every question that has it.
But a new user comes along who doesn't know the history and naively uses the useless tag.
Wouldn't it be handy if the system put up some kind of barrier to make this a bit harder.
Perhaps just a warning in red when you try to submit like we already have for questions with the word "best"... perhaps a warning in the place where tag wiki excerpts are displayed while typing in the tags section... perhaps some other way.
Of course sometimes a tag becomes unused for some other reason and recreating it later does make sense after all. So it probably shouldn't be completely blocked... at least not as the default - maybe for real problem tags.
The system could keep track of all the tags which were invented but later went out of use. For instance in the db there might be a "number of uses" field for each tag, which for these tags would be
0. When somebody tries to tag a question with one of these "
0 use tags" it shows the warning, or whatever action is deemed appropriate.
If the db doesn't currently remember which tags have gone out of use, one of the SE devs could write some script that finds them from dumps, backups, etc, and puts them in the db as "
0 use tags".
This is not asking about the difficult manual process of blacklisting tags (permanently blocking them, requiring an SE dev. This is about the system warning an editor that a tag might not be acceptable.
Both features can coexist but are not at all the same thing.