It's the Tuesday after a new Windows 10 Insider Preview was released in the Fast ring, meaning that it's time for a new SDK Insider Preview. The build number is the same, 18323.
The changelog is the same as it was for the last SDK. There's a new blend mode for AcrylicBrush called Luminosity, adding a new Fluent Design feature. There are also some new Message Compiler features that have been noted for some time.
As usual, this SDK works alongside previous SDKs, so you can use it in a production environment as long as your app still targets a previous version of Windows 10 when you submit it to the Store. Unfortunately, this build only works on Windows 10 19H1 machines on the Windows Insider Program, and it's only officially supported by Visual Studio 2017 and greater.