This article explains what S mode for Windows 10 is and how to permanently disable it.

Previously, switching out of S mode meant upgrading to a completely different version of Windows. However, Microsoft has recently made it possible to switch out of S mode completely with just a few clicks or taps.

Press Windows key+I to open Windows 10 Settings. You can also search for Settings in the Windows search bar and select the corresponding result.

Select Update & Security.