In human history cabbage has tended to be fermented or cooked rather than eaten raw, because it's the raw version that can deplete iodone. The brassicas are all in the same boat. But in moderate amounts on top of a balanced diet it's not a problem.
I feel it's a silly, over-cautious list. Everything is toxic, even oxygen. If you search for studies on artificial methionine (one of the staples of the commercial chicken food industry) you'll find evidence that it's associated with fatty liver, high triglycerides and cardiovascular disease. Fatty liver is a common cause of death among chickens. Somehow that doesn't count as 'toxic'?
You have to think about who does the studies and what kinds of interests call for what kinds of studies (while failing to research things that can't increase profit). Yes, much of what grows in the backyard is toxic in some mild way. A few plants are both dangerously toxic even in small doses, but not very many of these are also palatable. The ones that are both dangerously toxic and pleasant to eat are the baddies.
If birds are confined, they have more incentive to eat too much of something that's mildly harmful, which is why the list can sometimes be useful. Meanwhile keep on giving kale and cabbage, but not every day. A little seaweed will supply iodine!
Have a nice Christmas!
