At last year's Build, Microsoft announced Project Reunion, its latest attempt to bring the Universal Windows Platform (UWP) and Win32 apps together by decoupling APIs from the operating system itself and making them available to be used together without needing the latest version of Windows 10. Today, the initial release of Project Reunion is available as version 0.5.
This initial release comes with a few limitations, but it already has some significant groundwork done. It supports WinUI 3, WebView 2, and .NET 5, so you can already build desktop apps with all these technologies with Windows version support going all the way back to Windows 10 version 1809. With Project Reunion, you get access to Win32 features as well as "modern Windows technologies and features", which are usually designed around UWP.
The biggest limitations are that you can't build unpackaged apps with Project Reunion just yet, though support for them is on the roadmap for later this year. You can also only build traditional desktop apps, not UWP apps, and there's no support for multi-window apps - though these features are available in the current version 0.5 preview. Because this is a stable and supported release, some features aren't fully implemented yet, and you won't be able to reference them.
Microsoft has also highlighted a number of partnerships that will help integrate Project Reunion into other platforms. One of those is the Uno Platform, which allows developers to port WinUI-based apps to multiple platforms including WebAssembly, Android, and iOS.
To get started using Project Reunion, you'll need Visual Studio 16.10 Preview or newer, and the Project Reunion 0.5 extension. You can follow these steps to set things up. For those interested in the future of the platform, Microsoft also has a roadmap for 2021, which includes support for features like windowing and unpackaged apps by the end of the year.