On my 2018 F-150 Supercab's bumper, there is a plug with a slot for a key. It will come out using the pickup key, but there is nothing there except a hole. What is the purpose of this?

 

Its the opening you put the spare tire tools into to lower said spare tire, its locked so someone can't steal the tire.
Be aware the lock/plug has a top and bottom, you can force it in upside down but then its very hard to remove.