UPDATE: Access Control Service will be deprecated on November 7, 2018 and the ability to create new Access Control Service namespaces will be stopped on May 1, 2018 as part of this deprecation process. This includes previously whitelisted user Subscription IDs.
For additional information please take a look at this blog post.
For Service Bus, Event Hubs, and Relay customers that currently use Access Control Service (ACS) namespaces you can get guidance on how to migrate with the following articles:
Other migration guidance about ACS can be found here.
We highly suggest you begin planning and executing on a migration strategy today.