Stack.

Older and slower, as long as server meets these requirements:

https://blogs.technet.microsoft.com/server-cloud/2015/12/21/microsoft-azure-stack-hardware-requirements/

12 physical cores

96 GB RAM

Windows 2012 certified NIC

and more...

Not that people haven't already figured out how to bypass some of the checks. But that can be an iffy thing for production environments.