https://wccftech.com/microsoft-begins-kicking-out-insiders-running-windows-11-on-older-pcs/

After announcing the official release date for Windows 11 yesterday, Microsoft started pushing unsupported PCs running the Insider builds of Windows 11 back to Windows 10. The Windows maker had allowed some of the unsupported devices to run and test the new operating system. However, that ends now as you will see a message alerting you to reinstall Windows 10 if your device doesn't meet the minimum specs requirements of Windows 11.

Insiders in both the Dev and Beta Channels have reported seeing this warning.