If you delete a smart class parameter on a Foreman class and it has an override it does not delete entries from the lookup_values table or the lookup_keys table, so any hosts that the override applies to displays a parameter with no scope and it doesn't seem possible to delete it from the host edit page.

If you then add the smart class parameter back by doing an import it creates a new entry in the lookup_keys table that is associated in the environment_classes join table and the override is still inaccessible to delete.

From Ohad it seems the keys were intentionally left behind, but it would seem that leaving the values behind has an undesirable/confuding side effect in the interface, so it would probably be worth cleaning them up if the override they are associated with is deleted.