That's right, Visual Studio 2019 appears to have dropped support for JavaScript UWP projects, which is why we document that you have to use Visual Studio 2017 specifically. Usually dropping support in the latest release is the first step in retiring support completely. I would expect that eventually the Microsoft Store will stop accepting JavaScript UWP projects, at the latest by the time VS2017 reaches its end of life (probably by 2022 judging by the mainstream support period), since by then there will be no actively supported version of Visual Studio that supports JavaScript UWP projects.
This also raises the question about whether we should keep or remove the UWP export in Construct 3. They are by far the least used exporters. This always surprised me, given how much people talk about how much they want console support, and this being the only officially supported way to publish to Xbox One. It seems in the end hardly anyone actually uses it. I guess people are using publishers who do console ports instead. Frustratingly I've not been able to find any official line from Microsoft about this, so I don't know what their plans are. So I guess we keep it so long as VS2017 is supported and the Microsoft Store accepts its apps. But given this situation, it doesn't seem remotely worth the effort doing any further development work on it ourselves at all.