It has been announced sometime ago and now this is there the official timeline.
Azure Access Control Service (ACS) is going to be retired on November 7th, 2018.
It has been already removed from the Azure ARM portal earlier in April.
If you are still using the service you must migrate your ACS to one of the other authentication services like Azure AD, Azure AD B2C or other.
Microsoft has published some migration guidance available here https://docs.microsoft.com/en-us/azure/active-directory/develop/active-directory-acs-migration