Was ?
Windows 2000 could have just as easily been the OS that replaced the DOS-based Windows series, it was user-friendly and functional enough.
https://techcommunity.microsoft.com/t5/windows-os-platform-b...
However plenty of Azure infrastructure uses Linux as well,
https://github.com/microsoft/azurelinux
https://azure.microsoft.com/en-us/blog/sonic-the-networking-...
[1] - https://en.wikipedia.org/wiki/List_of_Microsoft_Windows_vers...