The tag untagged is applied to questions which either have all their tags burninated or to questions migrated from other sites whose tags from the origin site do not match any tags here. According to formal advice given by the SE team, this tag can be created at any time and should be edited out on any question that contains it ASAP so that they are properly categorized (otherwise they could just languish as noise).
This question is to serve as a formal tag cleanup request for the untagged tag and should indefinitely be marked status-planned even if the tag is clear, because it can be recreated at any time. Why am I asking this request? Because then it will show in the list of tags that need cleaning, because many users use this to bring up a full list of tags that need cleaning. I should also mention that in the old monthly tag cleanups, we also had an explicit warning against this tag.