There is no overview anymore -- Settings is always visualizing a panel at any given time. I just noticed that the Alt+Left shortcut was always broken too. This will be fixed in a following commit.
This explains the various keyboard shortcuts that are available in the control-center. See https://wiki.gnome.org/Initiatives/GnomeGoals/ShortcutWindows https://bugzilla.gnome.org/show_bug.cgi?id=759320