I don’t think this is possible. Maybe there was some issue in a previous version.
When you delete a Collection, it does not delete the contents of the Collection. Thus, any items in the deleted Collection that had not been previously assigned to a different (non-deleted) Collection will end up in the Unassigned pseudo-collection (it is not actually a real Collection, because you cannot configure its permissions or assign a vault item to “Unassigned”).
Thus the Unassigned pseudo-collection is only there as a catch-all, so that items don’t get lost when you delete the Collections that they belong to. Beyond that, it is possible to use the “Unassigned” status as a work-around for archiving vault items (storing items that you don’t want to see or use, but that you prefer not to delete); Bitwarden is currently in the process of implementing an “official” archive feature, though, so I expect this off-label usage to become obsolete…