Though some folks on bikes know what it means, most pedestrians have no clue what "On your left" means. Why should they? I too never heard it until I started being a cyclist.
Mostly what they hear is "mumble mumble LEFT!" anyway, and they obediently jump to their left.

Much better to call out "Bicycle!", "Nice day!" or "Passing you!". I think calling out just about anything is better than "On your left".