What is the impact of the “Requires full screen” option in Xcode for an iPhone-only app?

There is no impact at all. Apple engineers thinks that its not required to hide, or may be Plus phones will get landscape slide over in later iOS versions :)

From the documentation:

To opt out of being eligible to participate in Slide Over and Split View, add the UIRequiresFullScreen key to your Xcode project’s Info.plist file and apply the Boolean value YES.