Microsoft once again released a Windows 10 SDK Insider Preview today, bringing the build number to 17686. It aligns with the current Windows 10 client build in the Fast ring, which was released last Wednesday.
This new SDK doesn't really add anything in the way of new features. In fact, the big new feature came last week with build 17682, as that SDK added support for MSIX. MSIX is an app packaging format that combines MSI and APPX.
The only known issue for this build is that MSIX is still unsupported by the App Certification Kit and the Microsoft Store. This isn't surprising, and probably won't change until later this year. Remember, MSIX package only work on Windows 10 build 17682 or higher, meaning that the app will have to target Redstone 5, and Microsoft isn't accepting submissions for RS5 into the Store yet.
This SDK can be installed alongside of others though, which means that you can use it in a production environment as long as you target an older build before submitting to the Store. However, it will only install on Windows 10 Insider Preview builds.
To download the new SDK, you can find it here.