If you look at an OLED display off-angle, you might notice slight shifts in color and hue. This is a characteristic of OLED and is normal behavior. With extended long-term use, OLED displays can also show slight visual changes. This is also expected behavior and can include “image persistence” or “burn-in,” where the display shows a faint remnant of an image even after a new image appears on the screen. This can occur in more extreme cases such as when the same high contrast image is continuously displayed for prolonged periods of time. We’ve engineered the Super Retina display to be the best in the industry in reducing the effects of OLED "burn-in."
Apple offers the following suggestions on how to prevent these problems from occurring:
● Use Auto-Brightness to automatically adjust the brightness of your display based on the ambient light in your location. This setting is on by default. To check this setting, go to Settings > General > Accessibility > Display Accommodations.
● Set your iPhone X to turn off the display when you aren’t using it. Choosing a shorter time is recommended. To adjust this setting, go to Settings > Display & Brightness > Auto Lock.
● Avoid displaying static images at maximum brightness for long periods of time. If you have an app that keeps your display on when you aren’t actively using your iPhone X, you can temporarily reduce the brightness level using Control Center.
More details in the full support document linked below...