Microsoft officially announced the 21H1 version of Windows 10 last month. It is a small update that will upgrade the 20H2 version of Windows.
When Microsoft first announced the 21H1 version, Insiders on the Beta channel had the option of having the 21H1 build or the older 20H2. Now, all Windows Insiders in the Beta channel will be able to have version 21H1 as a suggested update.
Microsoft's Brandon LeBlanc described the change in the release notes for version 21H1 19043.844:
Update 3/2: 21H1 is now available as a recommended update to all Windows Insiders on the Beta channel.
As LeBlanc explained, if an Insider had chosen not to download 21H1 builds, it would start downloading them automatically via the Beta channel.
The build 21H1 is a relatively small update compared to some other half-yearly Windows 10 feature updates.
A much bigger update will be released later this year. Codenamed Sun Valley, the update will include an updated design, new features and updated applications. The biggest update is expected to be released in the second half of this year as Windows 10 version 21H2.