Deleting a series won't actually remove all traces of the Series. Deleting a series only makes it invisible as well as prevent any new purchases for that series.

For example, if a Studio is set up to grant New Customers a free series upon sign up, but that series ends up deleted in the future, customers will still receive the free series. You need to deselect the series in Studio configuration settings under "New Customer Free Series".

In other words, even when deleted, a series' traces are still alive in the system until they are removed throughout the system.