It's the mounting bracket for the included cooler on the card, it's there to maintain adequate pressure between the cooler and the GPU die so that it can efficiently transfer the heat to the cooler. You can get brackets to mount an all in one (asetek usually) cooler to the front of the card.
The first such mount I was aware of was the NZXT G10:
https://www.overclockers.co.uk/nzxt...fd4f5ea62b3-1627419630-0-gqNtZGzNAk2jcnBszRFi
I get that it looks similar to the intel cooler mounting, but that's because it serves the same purpose (pressure). Many cards have back-plates that cover this area but will have something similar underneath, back-plates that leave that area exposed are a recent design choice that a lot of brands are using, it may be that leaving it open allows passive heat dissipation from the chips on the back of the card, but it won't be much!