Apple this morning shared a new support document
explaining how OLED displays work and that it is normal to see some screen burn-in over time and shifts in color when looking at the iPhone display from an off-angle.
According to Apple, when you look at an OLED display from a side angle, you may see shifts in color and hue, something that's a "characteristic of OLED" and "normal behavior."
Apple says that with extended long-term use, OLED displays can show "slight visual changes," which is also considered normal. The iPhone X has been engineered to be the "best in industry" at reducing burn-in effects, but Apple's support document suggests burn-in is still a problem that some users could potentially see over time.
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."
With Apple referring to burn-in as normal behavior, it's not clear how this issue will be treated should it occur in terms of the one-year iPhone X warranty or extended AppleCare+ coverage. Typically, issues that Apple considers normal are not covered.
Apple's wording suggests screen burn-in is going to be a rare occurrence, but Apple does suggest users avoid displaying static images at maximum brightness for long periods of time. If there's an app that keeps the display on when the iPhone X is not in active use, the brightness level should be temporarily reduced using Control Center.
Making sure the iPhone X's display is set to go to sleep after a short period of time will also help prevent any burn-in issues, as it generally happens when the same image is on the display for a long time. Apple recommends setting Auto Lock to "a shorter time."