Creosote used to be the standard treatment for fences so I'm guessing the toxicity must reduce over time. I'd be inclined to leave the trellis somewhere out of the way where it can still get the weather on it. It will probably be usable by next year. You could test it by growing something like sweet peas up it. There'll be no great financial loss if it does kill them.
After 24 hours of not speaking to me except to say thank you for beer proffered, he is now making a new set of trellis. All I may add made form a very very old falling down shed that we dropped a year or two ago. So possibly creosoted many years ago!........but I won’t mention that!😉
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Very very old creosoted wood will be fine
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.