What does Shift-Command-Space keyboard shortcut do? Apple's macOS Human Interface Guidelines shows that action for the Shift-Command-Space keyboard shortcut is: Varies. Apple Reserved.

Does that mean that Apple is reserving it for future use?
On my iMac, running macOS High Sierra, it does nothing. Go to System Preferences > Keyboard > Shortcuts tab. This will list all the system defined and custom keyboard shortcuts. If there is something that you want Shift-Command-Space to do, you can define it here.

I just set Shift-Command-Space to Show Launchpad, and it works fine. So it appears that the key combination is currently unassigned.-

The image below shows where I defined Shift-Command-Space: