Historically, it feels more like a class thing. A proper, civilized person would keep their mouth closed. As for the current mouth vs nose and belly vs chest

Historically, it feels more like a class thing. A proper, civilized person would keep their mouth closed. As for the current mouth vs nose and belly vs chest breathing, it really feels like a fad/cult. There seem to be a constant stream of very small "studies" with questionable methodologies with just enough results to warrant further investigation.