"Users, ALL by Name" is a default custom picklist that does not allow filters. This picklist allows you to remove "Deleted" and "Inactive" users but if you deselect these options for the picklist the users will be hidden on items they are associated with. If you select these options, the picklist contains the full history of ALL users as options and becomes busy. If you could add filters to this picklist, you could remove the "Deleted" and "Inactive" users from the dropdown option without removing them from items they are previously associated with. If you have already utilized this picklist it would be a manual process to create a new, filterable picklist and repopulate ALL of the instances where it was used. Not worth the effort.
