Developer Steve Troughton-Smith spotted the code inside Apple's iOS simulator, noting that the code for it has likely existed for several years, even if it hasn't surfaced in iPhones yet. Big Android phone makers like Samsung and LG have offered truncated and even floating keyboard windows after the companies moved into smartphones larger than four inches. Android's native keyboard also has a one-handed option if you need it.
Troughton-Smith even released the code chunk for jail-breakers to make a reality... if you jailbreak your iPhone. And here it is in action, albeit steered with a mouse:
Video or it didn't happen: (very hard to engage in the Simulator with a mouse cursor) pic.twitter.com/vw2wpCgiLJ— Steve T-S (@stroughtonsmith) October 19, 2016