I heard that using low or high larynx is bad for your voice. But why is this true ? In what way is it bad for your voice? And is there a way to make low or high larynx healthy? Or should you at all costs avoid this? Or only use it in moderation?
High larynx puts your means of sound production close to the mouth. The sound tract can work somewhat like a horn speaker. In optimal conditions, your vocal folds operate against a quite larger air resistance than at the mouth opening. That allows them to transmit significant energy with comparatively limited movement, and vocal damage (and shrieking sound qualities) mainly comes from excessive displacement/movement.
The larynx resting in a low position is good for that. The larynx cramped into low position takes off energy again and adds strain (that also locks off natural vibrato which requires a relaxed larynx). Your mouth is better for sending sound than your neck is.