Over a month ago in Windows 10 build 17686, a "Switch to S mode" setting showed up. It never actually did anything - you could only find it by searching for it in the Settings app - and clicking on it would just bring you to the Windows Activation page. In today"s build 17713, that option has been removed.
If you"re not familiar with S mode, it"s a rebranded form of Windows 10 S, which would now be considered Windows 10 Pro in S mode. The rebranding allows for other versions of Windows 10 to have an S mode as well, such as Home and Enterprise.
When S mode was announced for the Windows 10 April 2018 Update, or version 1803, it was promised that users will be able to switch to full Windows 10 for free. It was never promised, however, that people would be able to switch back to S mode. After all, the process would likely require a factory reset to clear out all of the Win32 software. At the time, it seemed to be a common misconception that S mode would be a sort of toggle in Settings that you could turn on and off at will.
Back in February, Microsoft accidentally published a Feedback Hub Quest that provided steps for switching to S mode. The Quest was meant for internal use only, but we got to see it. It would seem that the "Switch to S mode" setting might have been a similar situation.
I"ve been digging around for more information about the setting, and haven"t yet been able to come up with anything conclusive. It would seem though, that if Microsoft will be providing the option to switch back to S mode without a recovery image for a device that shipped with it, it"s not coming in Redstone 5.