This is called a BS-1 by Nikon.
It slides into the flash grip on top of the camera and protects the contacts from water, muck etc.
I have just purchased a very expensive DSLR – and discovered that these things (which must cost about 1c to produce) are no longer supplied.
I thoroughly agree that it’s a good idea to limit waste, but this thing is actually useful – I could quite happily do without the umpteen layers of plastic bags, bubble wrap etc. inside the camera box, but this bit???