https://www.bleepingcomputer.com/news/microsoft/windows-10-20h2-kb5001391-cumulative-update-preview-released/

Microsoft has released the optional KB5001391 Preview cumulative update for Windows 10 2004 and Windows 10 20H2.

This cumulative update is Microsoft's April 2021 monthly "D" update that allows users and admins to preview the upcoming fixes scheduled for release in the forthcoming May 2021 Patch Tuesday.