That should technically work as well, since sugar also draws water out. It’s also a common tip in those recipes that use quick ‘caramelized’ onions, since it also browns faster than real caramelized onions. If you’re wanting to lower your sugar intake and aren’t worried about upping your salt (no high blood pressure, kidney problems, etc) then salt probably works better for drawing out moisture and getting cooked onions. But if you’re wanting the caramelized onion taste the trade off will be longer saute time to actually caramelize the sugars in the onion.