Yet another short note to my future self:
Running a Mac with a German keyboard layout (likely having the key sequence “7 8 9 0 ß ` <Backspace>” in the top row) and using a Parallels Desktop to run Windows and needing to enter a back slash (\) now & then?
Try the right-hand-side CRTL-ALT + ß (the key with the German ‘sz’ as the primary character).