* .appx became .msix; the latter also works for Win32 apps
* UWP XAML/GUI evolved into WinUI, v3 of which doesn't target UWP
* Win32 app isolation—this news—grew out of AppContainers, which were used by UWP apps
That said, what's new here? You could package (.msix) Win32 apps with partial trust, IIRC. Does this remove the need for packaging?
What is new here, is that this is the continuation to make all Win32 apps sandboxed.
Fair, I would say it's somewhat actively developed, for now... Which is maybe a step above UWP XAML.
Microsoft promised to open source WinUI 3 years ago, but it keeps getting pushed back in favour of other priorities. This isn't good for a healthy community, who can only report bugs, and never fix them.