On macOS, the most confortable key to be set as the sprinting one is fn, located at the bottom left corner of the keyboard. As not considered a functional key itself, in version 1.13 it's not responding when setting controls.

Players like me, who have been using this key for sprinting since lots of versions ago (1.7 in my case) are really suffering from having to change the position of our controls, as the in-game experience gets worse.