Well, turns out radian Emacs config provides an advice for such things:
(dolist (func '(define-minor-mode))
(advice-add func :around #'radian--advice-silence-messages))
which you can add to your init.local.el.
Here is its definition:
(defun radian--advice-silence-messages (func &rest args)
"Invoke FUNC with ARGS, silencing all messages. This is ...