How can I turn on "show open windows" dock element? I deleted this probably accidentally.
Go to Applications menu, right-click on Multitasking View -> Add to Dock.
If by any chance it's not there, create a file called gala-multitaskingview.desktop in /usr/share/applications/ with these contents:
[Desktop Entry] Type=Application Name=Multitasking View Comment=View all open windows and workspaces GenericName=Multitasking View Icon=multitasking-view Terminal=false Categories=GNOME;GTK;System; Exec=dbus-send --session --dest=org.pantheon.gala --print-reply /org/pantheon/gala org.pantheon.gala.PerformAction int32:1 X-AppStream-Ignore=true
Careful with that Exec line, make sure to copy all of it, it's quite long.
EDIT: Just remembered you could also assign the same functionality to one of the corners of your screen. Just go to System Settings -> Desktop -> Hot Corners, pick a corner and select an option from the drop-down menus.
There is also a keyboard shortcut:
WinKey + A or