Nope - your content will not be deleted if your Premium (or Team / Association) subscription expires. You will still be able to view the drills and practices you created. When your account lapses, you will only lose access to the Premium (or Team / Association) features - such as email, pdf export, etc. - but your content will remain intact. If you choose to upgrade your account again in the future, you can simply create a new subscription while you're logged in and you'll restore all the functionality.
Note that we do remove accounts that have not been accessed in the last 3 years. We do send email notification prior to actual removal of the account. To keep your account active, you simply need to log in to your account. It does not have to be Premium status in order to avoid removal.