I have never heard this either. I used to have a dog that got in my flower beds all the time, occasionally digging, but mostly sniffing and searching. Back then I had, lots more toxic plants, foxgloves etc, including lots of iris. I have cats too, none have ever bothered those plants. My cats play ricochet off the flowering iris in the spring
but no harm to them. Animals seem to know what is bad for them. I imagine the plants don't taste good enough to take more than a taste. And lots of times when you hear deadly toxic, a goodly amount has to be injested to cause death.