The hub as specified in the draft of 20 Oct 2016 can delete a subscription unilaterally from time to time:

• when a lease expires
• (perhaps) when a content distribution attempt fails "often enough" (see issue Failed content distribution attempts #15)
• (perhaps) in other situations?

It may be worth considering sending a notification of unsubscription when any of these events occur.

Such a notification could be similar to an unsubscription verification, except that the response from the subscriber would be ignored (and hub.challenge would be omitted, etc).