Shake to undo makes it easy to quickly undo typing on the iPhone, iPad and iPod touch. After shaking an iOS device, a confirmation dialog appears. For situations where the undo dialog gets in the way of other activities on the iPhone, the feature can be switched off. Shake to undo is enabled by default.
AirPods and headphones connected to the iPhone, iPad or iPod touch normally deliver audio in full stereo. In fact, even the external speakers on recent models offer stereo sound. However, there are times when mono audio may be preferred. One reason why iPhone owners might disable stereo would be to share AirPods or a single set of headphones between two people.
Back in the day, iOS always showed On/Off Labels on toggles, making various Settings appear like a power switch. More recent versions of iOS have removed this indicator, opting instead for a more clean-looking design. For those who could use an additional visual cue besides the color green, or simply enjoy the old look, it's possible to restore these labels.
While Apple has yet to offer an official "Dark Mode" setting for low light environments, iOS 11 does offer "Smart Invert Colors" in the Accessibility settings, which basically turns white backgrounds to black and black text to white (and vice-versa) and dims bright colors. This makes viewing in low light situations easier on the eyes and less distracting to others around you. Unfortunately it is not very easy to get to in a hurry because it is buried in Settings -> General -> Accessibility -> Display Accommodations -> Invert Colors -> Smart Invert.
When it counts, missing an incoming phone call on the iPhone can be a problem. While the volume can be cranked up to the maximum, there are situations where vibration can help. There's another option built into iOS that's designed to offer visual alerts. Incoming calls or alerts can trigger the LED flash on the back of the device to blink.