Anyway all I have is my own personal experiences with anxiety, and I can at least confirm that breathing plays a huge role in mood regulation along with physical posture, staying hydrated, and gut health.
Yes; one of the first things my therapist showed me after just talking about stuff was a breathing exercise, with the first thing being assuming a position that allows for easier breathing (lying down flat, relaxed, but feet propped up / knees in the air). "Relaxed" breathing is deep and slow breathing through the stomach, but you can't do that well if your posture is off and I can imagine this has long-term effects. Subtle, of course, and it's never one thing that causes anxiety etc.