When you downgrade to a free subscription, you continue to have access to all of your paid features until the end of your billing period. At that point, your subscription will revert to free. Your links will remain in your account and will not be removed.
If you have created custom links with a complimentary domain that was redeemed through us, the links will remain active for one year from the date the domain was first redeemed. After that, the links will become inactive.
If you purchased your own custom domain, that domain needs to remain within your account and the DNS record needs to continue pointing to the Bitly IP addresses in order for your links to remain active.