Although I'm reasonably experienced on Stack Exchange (8+ years of activity, 40 sites with 500+ reputation, 247k total rep) and have been a diamond moderator on one site for some 5 years, today I saw a tag-warning for the first time.
I have the impression this feature is used very, very little. Apparently, to get a tag warning, a Stack Exchange employee needs to act. That may explain why it's used so little. Stack Exchange employees are busy and many users, including moderators, will be unaware that tag warnings exist.
An example use I noticed on German Language Stack Exchange:
In this example, I guess the purpose of the tag warning is to get users to ask better questions. In the past, Earth Science accepted identification requests, and although the tag wiki contained detailed instructions on how to ask such questions, most users didn't see it and the instructions were rarely followed. In the end we banned such questions completely. Such a tag warning may have been useful to improve question quality, if effective. The only way to find out if they are effective is to try to use them, but then (1) I need to know they exist, and preferably (2) it should be little overhead to introduce them.
My feature request:
Please give diamond moderators the power to add tag warnings.
(Perhaps this power could even be reputation-earned, that could still be considered later.)