As of July 2017, you are no longer able to create a new ACS namespace through the Windows Azure Management Portal. To create a new namespace for Dynamics AX 2012 companion applications, open a technical support ticket for the Service Bus team to whitelist a new ACS namespace. You will then be engaged by Azure Customer Support to review the request. Please be ready to provide the subscription IDs you want whitelisted. For additional information, reference https://blogs.msdn.microsoft.com/servicebus/2017/06/01/upcoming-changes-to-acs-enabled-namespaces/.
The work to move Dynamics AX companion applications to SAS is in progress, and application updates will be released before the support for ACS is depreciated. Until then, the Azure support team has agreed to continue to support new AX 2012 customers with their whitelisting process. This whitelist process will end May 1st 2018.