We have been lucky enough to find Windows 10 Version 21H1 rolling out as a preview to our computer. Though the build number we have received is KB5001391, this is also the same build number that others are finding for a particular v20H2 update that rolled out at a similar time.
At the same time, we have received the Windows Feature Experience Pack for Windows 10 version 21H1 (build number KB5003345).
We have no news regarding what to expect in this update because we can’t yet find anyone else who has it. We will update this article when we read something. All we know so far is that some selected computers are receiving the 21H1 KB5001391 update while others are finding out ways to install it.
Update: We think this is likely a Windows Insider update, though we still aren’t sure what features we can find because of limited information.