Patrons should be notified if a hold becomes orphaned, i.e. there are no longer any items on the record which could someday fill the hold. A hold can be orphaned when items on the record are marked as lost, missing, reference, or any other non-holdable status, as well as when items are deleted.
Frequent library users may not want to receive notifications for holds that are ready, but may still want to be notified of orphaned holds, so it should be possible to subscribe to this type of notification but not the other.
The notification should contain a message from the patron's library suggesting alternative ways in which they might be able to obtain the item.