What is this black plastic square on motherboard?

This is an old Gigabyte Z170N-WIFI I have lying around and I have never been able to figure out what this black plastic square between the CPU and PCI slot is. Two corners have pushable pins (the backs of which are on the third picture) which suggests it is removable, though I haven't been able remove it by pushing the pins. I also find the grooves on it to be very strange. Does anyone know what it is?