Alongside of a new client build in the Fast ring and a new Windows Server 2019 build, Microsoft today released Windows 10 SDK Insider Preview build 17738. Just as you'd expect at this point, there's not a lot that's new. Redstone 5 will be at the RTM stage next month, meaning that development of new features is winding down.
Unfortunately, you still can't submit apps that target Windows 10 version 1809 to the Microsoft Store yet, which is why MSIX isn't supported by the Store or the App Certification Kit yet. That should be coming sooner rather than later. Luckily, you can still run the SDK side-by-side with your production SDK.
It's worth noting that this SDK will only work on Windows 10 Insider Preview builds, so you'll need to be on Redstone 5. Also, it's only officially supported with Visual Studio 2017.
You can expect to hear more about Windows 10 version 1809, the new version of Server, the SDK, and more at Microsoft's Ignite 2018 conference next month. In the meantime, you can download the new SDK here.