Once your subscription has reverted to free at the end of your billing period, your links will remain in your account and not be removed. If you have created custom links with a domain that was redeemed through us, the links will remain active for one year from the date the domain was first redeemed. After which, the links will become inactive.
If you utilized an external domain for your links, the domain would need to remain within your account and the DNS record will need to stay pointing to the Bitly IP addresses.