Deer aren't really the issue, agree on the pass-thru and probably minimal influence of deer hunting lead on the environment.
But, we shoot lots of coyotes, ground squirrels, and other little pest critters that are then eaten by others. The food chain is pretty unforgiving. For instance, we are finding that hawks and fishers and the like are dying off from the rat poison that the pot growers use to kill the wood rats that like to chew on the plant stems. The rats get sick and are easy pickings for those critters that eat them, then they get sick and die too.
The lead ban here started because of the California Condor. Why God made the critter so incredibly stupid, we can't know, but one of the few remaining birds died after drinking radiator fluid that had leaked out onto the ground. Why would any creature do that? The ban started in condor territory, after high lead levels were found in the hatchlings, who were dying after hatching or had thin shelled eggs or that kind of thing.