UWP Terminology

The Universal Windows Platform (UWP) has never been just one thing.
As a "platform" it incorporates many elements. There was the UI (which was a new "flavor" of XAML.) There were all the new platform APIs. And there was a new application isolation and packaging capability.
With a new operating system, designed to work on lots of different devices, you'd expect a lot of new functionality and capabilities.
Having a single term for all these capabilities that's different from the OS itself (which could still support old development capabilities as a way of maintaining backward compatibility) was a useful shorthand. "UWP" was that term.

UWP had some limitations as a term. That many people wanted "Universal" to mean any device or OS, not just Windows was one of them. Those people missed the second word in the term though. It meant "universal" across Windows devices. As a counterpoint, at the time Apple was using "Universal" to mean iOS apps built to run on both the iPhone and iPad.
There was also uncertainty as it was clear what made an app a UWP app. Even on Microsoft's own page explaining this, I don't think it's clear to be able to say if something is a UWP app or not.

Here's the thing though, it doesn't matter.
An app can be a UWP app regardless of the technology the UI is created with.
Any app which uses Windows 10 specific APIs can be a UWP app.
Any app can be packaged as a UWP app.

So, as the term loses its meaning, the need to use it diminishes as well.

WinUI covers everything to do with creating a modern, fluent, Windows 10 related interface.
Any app can use Windows [10] APIs.
And any app can be packaged with MSIX.

Maybe we'll need to use these terms in combination with "UWP" for a while (I've used "WinUI/UPW" to connect the two ideas in what I hope is just a transition phase) but hopefully not forever.
This isn't the "death of UWP", this is just an acknowledgment that the term isn't all that helpful and we can use other terms for more specific aspects instead. The underlying technologies aren't going away and there's no value in arguing over if an app is a UWP app or not.