It is open.
That is one reason so many quarters rust. Debris gets down there and collects moisure.
When I replaced my drop off on the drivers side I found:
3 drill bits
2 pieces of sandpaper
assorted wood and sheet metal screws
muck and dirt (just enough to hold moisture.
GM probably should have put a big flap down there to allow exit, similar to what is on the back inside edge of the rockers. Instead, there is a little tiny slit under the body plug.