Microsoft will force-update PCs running Windows 10 version 2004, as the OS version has reached EOL aka end of service. The Windows 10 version 20H1 aka version 2004 reached EOL on December 14. Microsoft clearly mentions in its official post that Windows update will automatically initiate a feature update on consumer devices running version 2004.
As of December 14, 2021, all editions of Windows 10, version 2004 and Windows Server, version 2004 have reached end of servicing. The December 2021 security update, released on December 14, is the last update available for devices running these versions of Windows.
To help keep you protected and productive, Windows Update will automatically initiate a feature update for Windows 10 consumer devices and non-managed business devices that are at, or within several months of reaching end of servicing.
This keeps your device supported and receiving monthly updates that are critical to security and ecosystem health. For these devices, you will be able to choose a convenient time for your device to restart and complete the update.
The December 2021 security update is the last official update available for PCs running Windows 10 version 2004. Microsoft has explained that all editions of Windows 10 version 2004 and Windows Server, version 2004 will cease to receive updates beyond the “end of service” date. Updating to newer Windows 10 versions like 20H2 and 21H1 is the only way to remain supported.