Most of the tannins and caffeine have been removed from steeping and spent tea leaves are then just extra vegetable matter. You have water extracted most of the caffeine, theobromine etc which is what I would be worried about causing problems for chickens. I feed spent tea leaves to my chickens regularly and I have never noticed erratic behavior, anemia, lethargy, or any other abnormalities. They enjoy them but pass on fine tea from teabags usually.