This has been an issue on my 2015 13' Macbook pro for a while, but it has gotten progressively worse over the months/years. I have no warranty left, but I plan on getting this checked out at an authorized service provider soon, as I'm aware that they offer free repairs for qualified customers.
Does this look like staingate to you? My concern is that because it is less damage than many other cases, that the authorized provider will reject it. If that happens, I might consider using some sort of Polywatch (like in this video) to scrub off all of the coating, and use some sort of anti-glare matte screen protector to compensate. What do you think?