Apple posts support page for iPhone X that advises how to reduce chances of burn-in on the screen
Apple also cautions iPhone X users that over the long term, the OLED screen can display signs of "burn-in." The company defines this as when "the display shows a faint remnant of an image even after a new image appears on the screen," and it happens in extreme cases when a high-contrast image is displayed on a continuous basis. Apple says that it has devised the Super Retina display to reduce burn-in.
There are advantages to using the OLED panel, and Apple points out that the one on the iPhone X offers a high contrast ratio and a high resolution. With the technology allowing Apple to skip the backlight, using the OLED display helped the tech titan make the tenth anniversary model thinner than it might have been otherwise.
The support page lists several suggestions on how to enhance the positives of using an OLED display with the iPhone X. These include making sure that the phone is always updated to the latest version of iOS, using Auto-Brightness, and having the screen shut off in the fastest time that you can live with. To lessen the chances of burn-in, iPhone X users should not allow static images to stay on the screen in maximum brightness for long periods of time.
source: Apple via SlashGear