The current tag excerpt of untagged is like so:
This is a special tag that is designed to be used only by the system for questions that have had all of their other tags removed. Do not add this tag to existing questions, it is reserved for use by the system to identify posts with no valid tags.
Currently there are 19 untagged questions onn Drupal.SE, that is: 19 questions with untagged as the only tag. Most (if not all) of these 19 questions have been explicitly retagged from some tag to the untagged tag, by moderators.
Some time ago (a few months?) there were around 100 of such untagged questions. Many of them I was able to cleanup via editing these questions and replacing the untagged tag with an applicable tag. The remaining 19 questions seem to be out-of-my-controle, hence this question ...