Only a handful of Windows 10 releases are still supported by Microsoft. And that's a big problem for people who refuse to update, as older iterations of Windows 10 no longer receive important security patches. To remedy the problem, Microsoft will force an update on customers who are still running Windows 10 version 20H1.
Microsoft is using a machine learning algorithm to roll out the first phase of its forced update. This algorithm will target PCs running Windows 10 version 20H1, which exits Microsoft's support cycle on May 10th of 2022. This is not new information---Microsoft shared the end-of-life date for Windows 10 20H1 upon its release, and the company announced the first step of its forced update plan in January of this year.
Just to clarify, Microsoft won't force you to update to Windows 11. The company will simply push a Windows 10 version 21H2 update to your PC. Microsoft plans to support the 21H2 release until June of 2023.
To check your Windows 10 build number, open Settings and head to the "About" tab. If you aren't running Windows 10 version 21H1 or 21H2, then you are no longer receiving security updates from Microsoft. I suggest updating manually to avoid a surprise update, which may be a bit inconvenient.