When a group is deleted, the keywords that were tracked there and are not tracked in any other regular group either, will be moved to an automatically created group, named “Ungrouped”.
Ungrouped has all the functionalities of a regular group, except for some of the Edit options. It only has Move and Archive (delete).
Once you archive all the keywords it contains, Ungrouped will be automatically hidden.
*Note: If a keyword is only associated with a Smart group, it will appear as Ungrouped.