Microsoft may be encouraging Win32 apps in the store, but is clearly saying that Win32 is undead and they'll continue to support the zombie and stop trying to kill it. They hope people will move on and they are providing more tools than ever to help migrate from Win32 to UWP. It's now a much more nuanced migration rather than a forced all-or-nothing conversion.
Wikipedia pages refer to everything in the past tense. That's the encyclopedia tone they require.
The closest thing to a "new UI kit" announced at the last BUILD was the ground up rewrite of the Windows version of React Native to make it "extra-Native". That change is from UWP/C#/XAML to UWP/C++/XAML. It's still JS controlling UWP controls, it's just using C++ to glue together the controls rather than C#/.NET.
The next closest announcement to a "new UI kit" was that way more of the UWP UI stack (almost all of it) is being "lifted and shifted" out of being directly inside the Windows codebase and deeply coupled to Windows releases and being moved in the WinUI library. WinUI has always been the parts of the UWP UI shipped outside of Windows Releases. UWP is moving to a model where they want to have as much as possible inside WinUI rather than inside Windows to make it easier to ship UI controls updates to application developers without so much of the rigmarole of "this control will only be in Windows 10 19H2 and later" making it easier for developers to use the controls "now" rather than waiting for a future Windows feature update and less headaches between what is in a current LTS feature update versus most current feature update.
(The next closest announcement after that, was that the Fluent Design team was merging the design efforts of what they've been doing for UWP/WinUI and what the Office team has been doing for the Web in the Fabric libraries, moving Fabric to being much more "Fluent Web". It should not be a surprise that the web is important to Microsoft's UIs.)
https://github.com/microsoft/microsoft-ui-xaml/blob/master/d...
The last GUI framework they entirely "left to rot" was Silverlight, and that was a) almost a decade ago now, and b) a fellow traveler in the XAML space where migrations were quite possible, if maybe not always easy/convenient, back to WPF or forward to UWP.
With the weeding of some of WinForms and WPF in recent .NET Core work, and the addition of XAML Islands, even what were the dead ends don't necessarily look so dead today, or at least not so rotten.