I have voted, however, Adobe need to first add a new feature to collapse a single selected/active layer group before a keyboard shortcut can be applied to the non-existent feature.
Photoshop only has an option for Collapse All Layer Groups. This isn't very useful or helpful if the user only wishes to collapse the active group.
For reference, the original discussion can be found here:
Where is it as a MENU ITEM? If it was there, we could assign a keyboard shortcut.
The only current menu item / keyboard command is to collapse ALL groups, not a single group.
Just because something can be done with the mouse doesn't mean that it naturally translates into other areas of programming.
Many things that can be done with a mouse can't be recorded into an Action or JavaScript code, which limits the possibilities outside of programming the application code itself.