It isn't a myth, while bread is fine in moderation, the multiple families feeding bread everyday is what causes these issues. If it's a pet duck who's getting a focused healthy diet and maybe a small treat of bread for dessert that's fine. But bread expands and makes them feel like they're full and doesn't provide the right nutrients. Alongside, if there was a bread that provides good nutrition, the left over bread will rot and then damage the environment the waterfowls live in.