Frying gets rid of the water in food, and salt is added to remove water from onions while frying. Since getting rid of water is the objective, if one has sufficient time, will it help reduce frying time or reduce the amount of oil sputtering if veggies like onion or bitter gourd are thinly sliced and left in the kitchen at room temperature for an hour or two to dry out a bit? I've noticed they get a bit wrinkled, which probably means they lost some water. Alternatively, chopping them and leaving them in the fridge in an open container seems to produce slightly similar results, but is it useful to prep it like this before frying?
Yes, it helps to dry veggies before frying them.
In fact, many recipes for fried and deep-fried vegetables contain explicit instructions for drying them, including salting, blotting between paper towels, or wiping down and dusting the surface with starch.
The reason why simply leaving veggies out on the counter is not a frequently seen instruction is that (a) that's a very slow way to dry them, and (b) it may cause them to oxidize or otherwise develop undesireable flavors or colors. But if you live in a low-humidity environment, there's no reason it wouldn't work.