I have already searched in the shortcuts in the Xcode preferences but I couldn\'t find a way to get it back.
My current interface:
Xcode 11 and 12: Click the "plus" button at the top, left of the "Show/Hide Code Review" button. Alternatively, use "View > Show Library" or hit Shift-Cmd-L.
Xcode 10: Click the "square in a circle" button at the top, left of the "show standard editor" button. Alternatively, use "View > Libraries > Show Library" or hit Shift-Cmd-L.