While searching for the Workspace Switcher shortcut keys, I discovered that you can switch workspaces by moving the mouse over the Workspace Switcher applet (in the bottom panel) and then scrolling the mouse wheel.
The following Workspace Switcher shortcut keys don't work when running Debian inside of VirtualBox:
I did find a workaround. Open Workspace Switcher preferences in the bottom right panel. Update Show all workspaces in: from 1 to 4 rows. The following Workspace Switcher shortcut keys now work: