You may keep using QBD Enterprise 2015 for good. But you are unable to reinstall it again any longer. They will decline to activate your license. You may have 2 options:
1. Upgrade to QBD 2021. You may purchase a new subscription with a perpetual discount for up to 20% off thru a partner.
2. Downgrade your file to be open on QBD Pro/Premier edition. You have to purchase a 3rd party service to do so. Contact us in private should you interest to pick this option.