Today, Microsoft announced that the Windows 10 May 2019 Update SDK is now generally available. That also means that if you're a developer, your apps targeting the new feature update can be submitted to the Microsoft Store.
Since Windows 10 version 1903 isn't generally available just yet, you can grab the SDK by going to the 'Individual components' section of the Visual Studio 2019 installer. Once the feature update is available to everyone, it will automatically be included in the 'Universal Windows Platform development' package.
The SDK includes XAML Islands v1, whereas the version for Windows 10 version 1809 was in preview. There are also improvements to the Windows Subsystem for Linux commands, and Windows UI Library 2.1.
The Windows 10 May 2019 Update is slated to ship in, you guessed it, May. It's available now to those in the Slow and Release Preview rings of the Windows Insider Program, as it has been for 10 days now. Presumably, you should be running the latest build for it if you plan on developing apps for it.