Broadly speaking, supportability means that Microsoft'll support certain products together. A case in point of supportability is Exchange Server 2010. While Microsoft supports it on some Windows Server OSs, it's not supported on Windows 2000 Server or client OS such as XP. Another example is Internet Explorer 8. It's supported on various releases of Windows, but isn't supported on older releases such as Windows NT.
"In this context, version 1.1 of .NET Framework isn't supported on Windows 7 and Windows Server 2008 R2. It was released in mid-2003 and was never intended to be installed or supported with products that weren't even in early planning stages at that time. Customers are encouraged to test their .NET apps with latest .NET Framework releases and update as appropriate.
In addition, we recently announced that later versions of .NET Framework (specifically .NET 3.5 SP1 and later) are now considered Components of Windows OS. Customers can now be certain of support for .NET Framework provided they're using a supported release of Windows," noted Microsoft.