Toxic is a bit of an overstatement. Irises, like almost all plants, produce a variety of chemicals intended to deter animals from eating them. But, a person would have to eat a lot of iris leaves to do any real harm (assuming it didn't inspire some sort of anaphylaxis). I suspect animals like deer, rabbits, etc. can handle those chemicals pretty well. Most of the time, there are other plants around they'd rather eat. But, it's also pretty clear that, at certain times of the year, iris leaves are their best option.