Most likely you are right, your display is affected by a burn-in effect.
The Galaxy S7 is equipped with an OLED display, this means that every small pixel is a small LED based on organic chemicals that can be individually lit.
The problem of OLED displays is that the longer and more often a pixel is lit, the pixel is aging and getting very slowly darker and darker.
Based the screen shot I would agree with you that this was caused by the Android task bar which is usually black (not lit pixels means no aging) whereas the rest of the screen above the task bar is usually not black and therefore is affected by the aging effect.
May be you often accidentally activate the screen when the phone is in your pocket (and therefore in portrait mode)?
Usually modern OLED displays should have implemented counter measures to reduce the effect that different parts of the screen are aging in different speed. However it looks like in your case this mechanism wasn't able to fully compensate this effect.
What you can do: The pixels aged pixels that have lost a bit of brightness can't be restored. Therefore the only way is to force the remaining usually black pixels and increase their aging.
This can be done by displaying an image that have all pixels black except for the area that is brighter (the area below the red arrows on your image). If you display this image on your screen for some hours a day you will see that the pixels will slowly will adapt their brightness to the pixels on the rest of the screen.