The current definition of this custom is as follows:
(defcustom sldb-initial-restart-limit 6 "Maximum number of restarts to display initially." :group 'slime-debugger :type 'integer)
Reading the way this is used, I believe this could be changed to
(defcustom sldb-initial-restart-limit 6 "Maximum number of restarts to display initially. Will always display all restarts if NIL." :group 'slime-debugger :type '(choice integer (const :tag "Show all" nil))
I don't venture to offer this as a patch because I'm not an expert with customize.
HtH, r