Iterating over global keymap, removing all bindings to super keys
Apart from using dolist as mentioned by @brfennpocock it looks mostly
fine.

system-type is a symbol though, so you can compare it with eq
instead of string-equal (unless I missed a reason to use that here).

Also the single if can also be when for consistency.

(defun global-unset-all-super-key ()
"Will unset any single key in global keymap that has the super modifier."
(dolist (km (current-global-map))
    (let ((maybe-event (and (listp (car km)) (caar km))))
    (when (and (eventp maybe-event)
                (memq 'super (event-modifiers maybe-event)))
        (global-unset-key (vector maybe-event))))))


For init.el if you have more functions also consider the
autoload
facilities, but that's not necessary for just this bit of code (actually
I'd probably put it all in init.el to be honest even though a separate
file like this is somewhat cleaner).