Is there a shortcut which invokes the "Login Window..." fast user switching command?
Control+Shift+Eject

or

Control+Shift+power-button

The latter doesn't seem to work in Mac OS X 10.11(.6), but the former still works.

To guarantee that the login window is shown immediately after entering one of the above shortcuts, make sure to change the relevant security setting to require a password (i.e. require logging-in) immediately "after sleep or screen saver begins":


Enter ⌘+space to open Spotlight search.
Type "security" and hit Enter when "security &amp; Privacy" is suggested; the "Security &amp; Privacy" settings should be shown.
Ensure you're on the "General" page.
Change the "Require password ..." setting to "immediately".


Source for info about the password security setting:


How to lock your Mac like you did on your PC - fixedByVonnie