I think there is a bug in XFCE. If I go to Settings > Window Manager > Keyboard and assign the shortcut Super + D it works until the next restart. In order for this shortcut to work always, you need to first remove the existing shortcut assignment. So, first hit the button [Clear] to remove the default shortcut and then hit the Super + D combination. Otherwise, the existing shortcut remains "under cover" and interferes.
Also, of course, you may want to clear any other places where the Windows Key (Super) is used, e.g. to show Whisker Menu, because, otherwise every time you hit Super + D it will try to open Whisker Menu. For example, I use other combinations with the Windows Key, like Super + T = launch Terminal, Super + E = launch Thunar (analogy of Super + E on Windows, where it launches Windows Explorer). So, I had to clear the assignment of the Win Key to show Whisker Menu to prevent the menu from always popping up.