In the manual, it says "If your serial subscription has expired you won’t be able to receive issues." But that is not actually true. Expired subscriptions can still generate new issues and you can still receive them. To stop a subscription from generating new issues, you have to close it.
However, it appears expired subscriptions cannot create items (and don't give any clear explanation of why the idea will not be created).
Filed bug 31268 for the issue I described with item creation.
I think for a subscription that is expired, but where not all issues in the subscription lifespan have arrived yet, it makes sense to still be able to receive issues. But we should probably not be able to create some with prediction/publication date after expiration date?