I have also seen this strange idea before. I think it is nonsense except for the long straight bang which may obstruct the vision. However, other types of bang, such as tilted bang or divided bang, have no effect on the eye health. So just don't make your bang leave so long that cover the eyes. It will not have bad influence on your eye health.
In fact, it depends on what kind of bang you leave. People say that bangs are bad for eye health, I think they mean if your bangs are so long to hide your sight, of course, and it's bad for your eye health. But you can leave divided bang or titled bang, these bangs won't hide your sights and look cute, too. So it's good for you to leave bangs, just don't hide your sights. I am sure you will look prettier to leave bangs.
Hair shade obscures an eye, just like wearing a mask. It will restrict stimulation access to the eyes of sunshine and sensory. Moreover, in the actual life, too long bang has the same principle with coarse glass, the normal eyesight person is easy t to get myopia, and the original myope eyes will become high myopia. Moreover, bang too long will also cause obscured by the eyes amblyopia, even wore glasses correction, the eyesight is still not more than 0.8. So, in my opinion, if you want to leave bangs, you canâ€™t make it too long.