Deep dives into the "iOS Simulator" embedded in Xcode has found code for a one-handed keyboard implementation dating back to iOS 8 that remains unfinished, and unimplemented by Apple.
Developer Steve Troughton-Smith discovered the code in the simulator, which is used to test iOS apps before they are loaded on actual hardware. The one-handed keyboard seems to be originally intended to be eabled by a left or right swipe from the edge of the keyboard to pull the keys to one side or another.
The keys clumped on one side would allow for easier one-handed data entry, particularly of use for Apple's larger iPhones. Troughton-Smith has also provided a video to demonstrate the invocation of the keyboard.
While the code fragments exist in Xcode, Apple has never implemented it. Troughton-Smith has provided code for enterprising coders in a jailbroken environment to implement the feature, at least partially.