"Supported" in a sense of patching security holes, sure. Not so much in a sense of writing new apps in it, or using modern tools to maintain old apps (support was dropped in VS 2017). So in that regard it's very unlike MFC or WinForms, which both enjoy continuous tooling support, and fixes and minor enhancements outside of security.
FWIW, I think your criticism is broadly valid, in that it's more of an exception for Microsoft, but much more common for Google. It's not hard to look at the Windows dev ecosystem, and see which parts of it are very stable, and likely to be around long term - because they have already been around for very long indeed.
I think the difference between MFC and WinForms, compared to Silverlight is that former are still widely used. albeit less than in the past with a lot more prevalence of web based applications.
Silverlight in other hand is a technology no modern web browsers support (in the same camp with gradually deprecated Flash and everything that requires plugins.), and perhaps to Microsoft it's a lot less justifiable to update it beyond security update.
FWIW, I think your criticism is broadly valid, in that it's more of an exception for Microsoft, but much more common for Google. It's not hard to look at the Windows dev ecosystem, and see which parts of it are very stable, and likely to be around long term - because they have already been around for very long indeed.