On macOS, is there a way to have releasing Cmd (just that one key) say open Spotlight? Similar to tapping the Super key on Gnome in Linux.
On Linux, key release triggers bindings, so its no problem to bind to a modifier key like the Super key or Command key. I believe macOS binds to key presses, so this may not be possible. Unless someone has a way around that?
On Linux, key release triggers bindings, so its no problem to bind to a modifier key like the Super key or Command key.
There’s a free utility called xev that, unfortunately can only be run in XQuartz (when I find a native app, I’ll test and post that). When running this utility, it will report the scan codes that are generated from each key press.
From a previous answer I wrote entitled How do I get the calculator button on a Microsoft Sculpt keyboard working?, we can see that the command (not specifically ⌘ Command) is bound to the release state of the key.
On macOS, is there a way to have releasing Cmd (just that one key) say open Spotlight? Similar to tapping the Super key on Gnome in Linux.
I also don’t recommend binding UI functions (i.e. Spotlight) to a single modifier. I’m not a Linux user (prefer FreeBSD) but I am familiar with it. The “super key” is a Linux convention and is usually mapped to the unused ⊞ Win on a PC keyboard or the ⌘ Command on a Mac (booted to Linux).
As macOS already has this key assigned, mapping additional functionality to it’s individual pressed state, can only cause problems.
I think the feature you're looking for is called Sticky Keys.
From Change Keyboard settings for accessibility on Mac:
Press a set of modifier keys one at a time in sequence instead of all at once. For example, enter the key combination Option-Command-F5 by pressing each key in sequence.
By default, Sticky Keys can be enabled by pressing the Shift key five times. You can modify this as well as other parameters by clicking on the info button 
.
Note : You can also enable Sticky Keys by going to System Settings > Accessibility > Keyboard and enabling Sticky Keys.
Cmd-Spaceto launch Spotlight. The problem with binding to a single key, especially to just the press or release, is that humans are imperfect and this will trigger all sorts of unwanted behaviors.