It was technically a Bellamy salute (https://en.wikipedia.org/wiki/Bellamy_salute), which the Americans invented and used a lot before WW2. Not that this excuses anything, it is still out of style.
Specifically, it went out of style because it was easily confused with that Nazi salute...according to your own link.
Yes, “before WW2” should have made that obvious. I’m pretty sure this is common knowledge.
No need to get defensive! Lots of things that are causally unrelated to WW2 happened before it, and there are many people around trying to excuse Musk's Nazi salute as something that it is not, including as a "Bellamy Salute" or "Roman Salute"