A very useful tip. I won't even plant a black walnut on the property 'cause one of the oldtimers around here told me I shouldn't. Not sure how far away it would have to be.
From observation, anything outside the dripline is usually safe, as long as the leaves aren’t used as mulch, of course. The leaves don’t have much of that chemical in them, but enough of them piled on can have an effect.
I read several articles on this, and they said fifty feet from the drip line for things that are sensitive like Tomatoes.
Corn, beans, and wheat are supposedly tolerant to the walnut toxin, and can be closer. We have 2 garden patches that are next to walnut trees. So far, the corn beans and wheat crops have been pretty good in these patches.