@PixelOrange got it mostly right from my research, though their backgrounds will still have that bar. If you use a black, or near black, background, especially one not at your native resolution, that gradient will be most noticeable. Using other backgrounds that do not have a black strip along the top you might still be able to see it if you squint.
For those of you curious: Change your background to a solid black from the background menu in settings, or grab something similar elsewhere. Then crank your brightness up. Chances are it'll be there for you too if you're not running a third party launcher.
I haven't found a way to turn it off in native yet.