I learned it in a RedCross program; which makes me think it was standard.

I suspect that it was never taught for competition, partly because of its energy efficiency through low performance.

I also wonder if it's more common around lakes and oceans; as it's great for going over waves with minimum energy.

I don't think that I was ever explicitly taught it as a child but at some point I stumbled across it by accident it's one of my favorite ways to swim casually. As someone else mentioned above, it totally feels like a jellyfish swim :)