The most up to date readme can be found here https://github.com/jtbrower/WinUI/blob/master/src/README.md
We learned that the September pre-release for WinUI had been cancelled. This followed an already pressing timeline where no commitment had been made to a specific production release date, no Visual Studio XAML support, no intellisense and a long list of other missing features where Microsoft provided no promise on whether they would be implemented. Its just too early for WinUI and too risky to plan on using it in any production application in 2020 let alone 2021.
You will still find a lot of great tips and tricks in the libraries that I developed. For now, it appears that WPF is still the most robust, hardened and predictable way to build Window's applications that operate outside of a Sandboxed environment.