Introduction:
After having installed Linux Mint 19.3 on an old MacBook Pro, I wanted to disable the Menu pop-up function on the ‘Left Command’ key press.

Solution:
Delete the following entry in the Settings Editor(in the settings menu):
xfce4-keyboard-shortcuts –>> /commands/custom/Super_L | xfce4-popup-wiskermenu

Calveat:
First I could not delete the command. Then I simply deleted its value content: xfce4-popup-wiskermenu
Then I could click on the Super_L and click on the Reset button at the bottom.
The entry disappeared and the menu didn’t come up any more at the press of the Left Command key.