Internet Explorer on Macs back in the days of the IE / Navigator wars. Lotus document formats back before MS Office was the de facto standard.

And even just talking about .NET, that was only created to compete with Java after Sun to Microsoft to court over MS's own Java implementation (though granted it's since evolved into something much more). .NET was originally sold as but MS quickly lost interest in pushing it on non-Windows platforms. And then came Silverlight to compete with Flash; and the cross platform hopes for that followed a similar fate once it became obvious that Flash was no longer a competition.

But as I said in another comment, it's a common enough strategy - hardly something unique to Microsoft. It even has it's own coined term: Embrace, extend and extinguish https://en.wikipedia.org/wiki/Embrace,_extend_and_extinguish

Windows Media Player for Solaris during media wars. When they ended, support was dropped.

Internet Explorer for UNIX (Solaris, HPUX) during browser wars. When they ended, support was dropped.

Does leopard change its spots? If they are successful with ASP.NET as The Web Framework (not much realistic, I know), expect them to drop support for anything but Windows.

Windows NT did have "subsystems" for POSIX and OS/2 along with Win32 back in the day.