SUA was dropped for Windows 8.1 and Windows Server 2012 R2.
Before that it was available in some form or different name in higher SKUs of Windows and in Windows Server.
Didn't help that MS kept changing what provided Unix support - from MKS Toolkit to Interix (which they bought out). What killed it I think is virtual machines. No longer need POSIX compatibility in the OS to support it on a server as it can instead done in a VM running a variant of Unix.