If you cancel your subscription, Apple will stop billing you for Redirekt Pro from the next billing month. Your Library and Reading List items will remain as they are, but you won't be able to add new links, if you exceeded the limits of Redirekt Free.
We will never delete your data, unless you explicitly request to do so.
Please note that your subscription will be considered as cancelled also in case of billing issues. After the 15th day of the next billing period, if Apple was unable to bill you for Redirekt Pro, your account will be downgraded to Redirekt Free.