Starting this fall with the release of iOS 11, iPhone users will have the option of invoking a special one-handed keyboard, making it especially easy to type with larger screens.
In an iPhone in portrait orientation, users just need to hold down the emoji, or globe, button to bring up the keyboard selector, as with any other keyboard.
Underneath the list of available keyboards, are orientation icons. Tapping on one of those compresses the keyboard, and snaps it to one side or another, or enlarges it to full width, and re-centers it.
Users accustomed to a two-hand grip on an iPhone, particularly the iPhone 7 Plus, could probably use a bit of practice to get back up to speed on the left or right-oriented keyboard, considering that you're using one digit. However, the ability to use the phone one-handed is positive for accessibility concerns, and also for other uses necessitating a hand that would be tied up in entering text otherwise.
Code snippets in Xcode were discovered in October 2016. The "iOS Simulator" was found to have a similar implementation dating back to iOS 8.
The one-handed keyboard seems to be originally intended to be enabled by a left or right swipe from the edge of the keyboard to pull the keys to one side or another. Apple apparently steered away from a swipe at some point, with the aforementioned press and hold of the Emoji/Globe button needed to invoke the keyboard selector.
For more, see AppleInsider's ongoing Inside iOS 11 series.