2.0 - If a subscription with active subscribers is deleted, this is what's supposed to happen:
"If you delete this subscription all users currently subscribed will lose any access rights granted by the subscription. Unless you are sure you want to do this it is recommended that you simply deactivate a subscription rather than delete it."
Sadly, what actually happens is a bit different. The subscription is deleted from paid subs, and from member's profile, but the member remains in the subscription membergroup. Since a subscription with an expiration date no longer exists, the member remains in that group indefinitely. Admin cannot manually cancel the sub because it's not there - the subscription membergroup is no longer linked to a subscription. Subscriber must be moved back to their original group in Profile -> Account Settings.
Moved to https://github.com/SimpleMachines/SMF2.1/issues/1166 where it can be tracked properly.