My assumption is that it's probably been a thing since roads, so possibly since at the least Ancient China, but I don't know, so I was curious if any experts know.

Not roadkill per se, but most ancient societies had laws governing accidents caused by draft animals and carts. But I’d imagine they mostly took place in crowded urban streets, not rural roads with wild animals.

Given the data on human survival rates in collisions, high rate of speed is almost completely the causal factor of serious injury and death. So, at least for larger animals I would say it's a modern phenomenon. Maybe some rats or toads or something got hit by a wagon wheel but the scale would be nothing like today.