The chemical does not persist in the soil for more than a year after removal of the tree.

Here is a link that might be useful: Walnut Toxicity

I have some year old walnut boards that I wanted to use to construct raised beds for vegetables. Is there toxicity in the boards that would affect growing vegetables, or is the toxicity limited to the roots of the walnut tree?

Walnut boards for making a raised bed??? Please say you are joking!...give those old boards to a wood worker and I'll bet he will trade you for something else!!
But I don't know it there would be toxic stuff in the boards....but don't think so.
Linda c