Windows 10 Preview Build 18312 is now available for Insiders in the Fast Ring. It comes with Reserved Storage implementation, improved Reset page UI, Windows Subsystem for Linux Command Line Tool improvements. Overall, this build is more about general improvements and fixes as Microsoft appears to be focusing more on stabilization of the OS. Microsoft says that implementation of Reserved Storage in Windows 10 is supposed to improve the day-to-day function of your PC by ensuring critical OS functions always have access to disk space.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-build-18312-19h1-released-with-reserved-storage-implementation/