This article from January 2018 states that AKS will be coming to Azure Government soon. Do we know of a target delivery date?
An older post references a target of Q3, but the newer post was more vague.
The latest and most accurate update on timing of feature availability in Azure Government is always the Azure Government feedback forum.
As such, you should go by the date posted in the Azure Container Service (ACS/AKS) in Azure Government feedback entry which at this point in time is preview in CY18Q3. Vote for the feedback entry so that you get updates as they become available.
Side note: The Trends in Government Software Developers article dates back to January 2018 whereas the latest response to the dates back to February 2018.