https://arstechnica.com/gadgets/2018/03/switching-from-windows-10-s-to-regular-windows-will-be-free-for-everyone/

Last month, leaked documents revealed that Microsoft would be making Windows 10 S, the restricted version of Windows 10 that can only run applications installed from the Microsoft Store, a mode of Windows rather than a separate version. That change is now official, and Joe Belfiore, corporate vice president for Windows, has provided further details on how S Mode will work in the future. It will be available to all desktop versions of Windows, and removing the 10 S restrictions will be free for all.

Currently, Windows 10 S is treated as a distinct Windows variant. Functionally, it's a specially pre-configured version of Windows 10 Pro, using existing Windows features to restrict it to only being able to use Store apps and to block certain built-in programs such as the command-line and PowerShell. Because of this close similarity to Windows 10 Pro, Microsoft has offered an upgrade from 10 S to 10 Pro for those who want to lift the restrictions. On some systems this upgrade has been offered for free (albeit only for a promotional period); on others, it's a $50 upgrade.