OLED screens are made of tiny organic led diodes for the pixels. Because these LEDs are made with organic material, they are subject to degradation and wear over time. Put simply, if one part of the screen is bright (or on), while other parts are turned dark (turned off or dim), the pixels will wear unevenly. The part of the screen that was bright will appear darker all of a sudden when what is supposed to be an even colored background is displayed. This is easiest to see on darker gray images, such as the dark theme in the Discord app (where I noticed it first).
Requirements:
You need a keyboard that can be themed (hopefully the one that is retained on your screen is one of these), such a GBoard. If not, you can still attempt this.
Inverse Theme:
Say in your current theme has black keys with white letters on a dark background. You need to find a theme that's as close to the opposite of that as possible. In this case white keys, dark letters, on a light background. Use this for a while until the retained image fades to where you can barely notice it, or is apparently gone.
Check every once in a while, as the opposite issue can happen if it's used for too long. High contrast keyboards make the issue worse and set in faster. Though they can be mostly reversed and are temporary most of the time.
If you can't get an inverse theme:
You can take a screenshot with your keyboard visible. Then invert the colors. You should edit the rest of the screen to be close to the same color as the keyboard when it's inversed (if the inverse keyboard is white, then make the rest of the screen that same color, or slightly darker. You want the pixels the wear as evenly as possible obviously.
Simply leave this image on the screen for an extended period of time. It may help to turn the brightness up to near maximum. Though remember, you are still wearing the pixels. Even though the screen will take an extremely long time to dim noticably, it's still worth noting. Check on it after a bit. Do not let it do this for hours or overnight. Try 5 or 10 minute increments, especially if you're using a high brightness.
Prevention:
After you got to the point where the retained image is barely or no longer visible, you NEED to switch to a lower contrast theme. The colors should be closer to each other than your previous theme. Also, make sure it isn't pastel or too bright of an overall color.
Personally, I use a theme with a darker blue background, overlayed with slightly lighter blue keys, and white letters. Removing key borders may help slightly too, as it will reduce what can be burned in.
Extra info:
+ There exists a point of no return when it comes to burn-in. This point is where the damage can no longer be "reversed" via this method. It can become permanent or infeasible to repair.
