It’s a standard A13 feature. You’ll find it in System->Gestures->One-handed mode. It pulls the screen halfway down so that you can easily reach the top half of the display still with one hand.
Also, the default Google keyboard allows a floating mode or a one-handed mode where you can make the keyboard smaller so that your thumb reaches all keys. But it’s not really practical for me. I’d much rather have the standard 12-button keyboard of old phones 