Once upon a time wood was treated against termites and fungus using arsenic. Those days are gone. Modern treatment is with copper sulfide. Still not something you want your rabbits eating daily, but much less inimical to health. Not only did I use treated lumber in the hutch, both where rabbits can chew it and outside the wire, but I painted it with a semi-solid stain.
My rabbits have chewed on the wood on occasion. As someone pointed out is a different thread, it might depend on the rabbit and conditions. I built the hutch with plenty of room to move (the base apartment is 4 feet long expanded to 8 feet) I have chew toys in there and I give her a lot of wild forage including dry leaves and bramble branches.