If you are using Dark Mode for a while. Now go to System Settings -> General, choose Hide System Menu Bar Automatically, then change your desktop wallpaper to pure stone color (the color is predefined in wallpaper app). Now, can you see the screen burning? Mine is taken by an iPhone 6s Plus from an iMac 5K 2014 late. On the bright side, if you change back to Light mode, after some time, the burning will get better. However, I don't know if it will be cured at last.