Windows 8 does not support C++/CLI.
It is the other way around, the support does not come from the Windows team (they did not ship a compiler) but from the Visual Studio team. Each Visual Studio version has a list of supported platforms that your application can target if you use that version of Visual Studio to build your app.
So does Microsoft support writing C++/CLI apps for Windows 8 Desktop? The answer is yes if you use the right version of Visual Studio*. Does the support come from Windows? The answer is no.
*You can continue test and provide support to your customers even without Microsoft's support to you, for example you can write C++/CLI code in Notepad then compile it using the cl.exe from VS2012 Express for Windows 8. There's really nothing to prevent this use case to work, despite Microsoft's statement that VS2012 Express for Windows 8 does not support desktop development (and I don't think there's any reason why Microsoft should provide support on this use case with VS2012 Express for Windows Desktop out there) .