As far as I understand, empty tags should be removed after 24h. But right now, the tags geometric-modeling and cycle-index are still around even though I'm 100% certain that they have been empty for much more than 24h. There is also chain-rule, but I don't know for sure that it has been emptied less than 24h ago. What is going on?
Even stranger is the case of untagged. This question was migrated twice, once to main then back to meta. When it was migrated to main, since the tag discussion doesn't exist there, it was marked untagged. When it was migrated back, it apparently stayed around somewhere in the DB and fills up the tag untagged (see for yourself), which cannot disappear now.