Today, Microsoft released a new Windows 10 SDK Insider Preview, build 17069. The new SDK coincides with Windows Server build 17074, which was released earlier today, and Windows 10 Insider Preview build 17074, which was offered to the Fast ring last week.
Obviously, new SDKs can't be pushed out with a higher build number than the actual version of Windows that they'll be running on. In other words, while the build number of the SDK still isn't quite as high as the current Fast ring build, it's still higher than the previous one, which was 17063.
The new Windows SDK is only formally supported by Visual Studio 2017 or later, but the good news is that it can work side-by-side with older SDKs. You shouldn't have a problem running this is a production environment, as long as you're targeting your apps at a production version of Windows 10.
Microsoft hasn't published a blog post just yet outlining what's new, although for this feature update, the big new feature is support for C++/WinRT compilers in the Windows SDK. Another thing that we'll likely see in the known issues is that this can only be installed on a PC running a Windows 10 Insider Program build.
Update: Microsoft has now published the release notes for this build.