They don’t really become poisonous. Of course it is best to cut vegetables and fruits fresh and any thing cut much ahed of time has the potential for germs or bacteria to grow.
I don’t intentionally cut onions ahead of time as they do release their gases but at times when I have a bigger than required onion, I do keep the peeled and/or chopped onion in an air tight container in the refrigerator and use it by next day. I have never kept cut onions for more than a day and never used if I left them outside on the counter and forgot to keep in fridge.
That is the best answer from my experience.