Windows 10 19H1 Build 18343 is now rolling out to the Windows Insiders in the Fast Ring with improvements for Windows Sandbox and general bug fixes. The update can be downloaded by navigating to the Settings app on Insider machines. Microsoft says that they have locked down the inbox apps and some features have disappeared from apps like Photos like Photos. The changes included in this release will be shipped to the public with Windows 10 April 2019 Update. Microsoft is now finalizing the April 2019 update.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-build-18343-released-to-insiders-with-windows-sandbox-improvements/