I'm pretty sure Nielsen already tested it, if I had to guess they probably found that different icon shapes are broadly better but that gets ignored because "it

I'm pretty sure Nielsen already tested it, if I had to guess they probably found that different icon shapes are broadly better but that gets ignored because "it's cheaper to use some shitty vector squiggle in a round rect", just like the research that found "icons are better when there is text" is widely ignored too because internationalization costs money.