most of the time we cannot grab just modifiers (Super, Ctrl, Alt, etc.) because grabbing a modifier key would forbid any keybindings with that modifier key. this is   a inherent deficiency in X keyboard grabbing.

Actually there's a workaround to this problem. but it dictates that only one keybinding programs.  In deepin, serveral programs support keybindings, e.g compiz, gnome-settings-daemon(we add a keybinding plugin here). so it's infeasible to bind Super key now.