Microsoft has started auto-upgrading a larger number of Windows 10 devices nearing end of life to Windows 10 21H1. Earlier this month, Microsoft began training their machine learning algorithms by upgrading a small amount of. Windows 10 2004 users will automatically be upgraded to the latest version of the Windows 10 version. Microsoft did this training to discover issues that may arise during the update process before upgrading a larger population of users. For most people, Windows 10 is still being offered as an optional update displayed when manually performing a ‘Check for updates’ in Windows Update.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-begins-autoupdating-more-devices-to-windows-10-21h1/