Certain slots may be hidden per default, or may have become "undefined" when the extension defining them was disabled. The content of such slots should still be accessible somehow.
The suggested mechanism is to introduce a slot=xyz URL parameter for the view action, that causes the content of the given slot to be displayed alone, regardless of whether the slot role has a SlotRoleHandler defined.
Links to these per-slot views could be given from a list of all slots present, from action=info. Though does not provide a way to access all slots of old revisions, we still need a solution for that.