In today's new Windows Insider Preview, build 17713, the Switch to S mode setting that was spotted in the last few builds has been removed, so the feature likely won't show up in Redstone 5.
Switch to s mode RSS
Yesterday, Microsoft released Windows 10 build 17686 to the Fast ring, and there's a new Settings option, which says 'Switch to S mode'. It's just a teaser though, as it doesn't do anything.
