Racket also optionally supports a square bracket syntax for cons clauses.

For example, in traditional Scheme one would write:

  ; this is in R5RS Scheme
  (define (length elems)
    (cond ((null? elems) 0)
          (else (+ 1 (length (cdr elems))))))

But in Racket one would write:

  ; this is in Racket
  (define (length elems)
    (cond [(empty? elems) 0]
          [else (+ 1 (length (rest elems)))]))

It is also used in let/let*/letrec bindings in Racket; see https://docs.racket-lang.org/guide/syntax-overview.html#%28p....

I don't know the origins of when brackets started appearing in Lisps. I don't remember seeing brackets in the LISP 1.5 Programmers Manual when using S-expressions (though brackets are a core part of M-expression syntax, which LISP 1.5 still supported), and I haven't encountered brackets in Common Lisp.