Anthropologically speaking, when humans have come across an animal that had no useful purpose to us, and that was a threat to us in some way, directly or indirectly, we have hunted that animal to extinction if at all possible. It's what we do.
One has only to look at the British Isles for proof of that; bears, wolves and large cats were all extinct on the islands as of the 16th Century, with bears being one of the top priorities of medieval Celts. Bears are only barely trainable if you raise them from cubs, you definitely wouldn't call them domesticated in that state any more than you'd say the same for a cougar or tiger in a zoo, and the meat of most predators went out of fashion as a foodstuff in Western culture centuries ago (though it remains a delicacy among native American nations and in East Asia). And the Eurasian brown bear, close cousin to American subspecies like the Grizzly and Kodiak, is more than a match for even the strongest humans in close combat.
So, the Celtic Britons made it a point to kill every one they found, which resulted in the Eurasian brown bear's extinction throughout the British Isles by the 10th Century. Wolves and lynx were lower priorities, more a threat to cattle and sheep than humans themselves, but that threat was still sufficient to cause those species' extermination on the Isles by the Tudor dynasty. The Brits and Irish had subsequently made good headway on the next lower tier of predators, the fox, badger and wildcat, before conservation mentalities gained steam in the late 1800s.
Exterminating similar species on the European mainland and in North America was a much bigger ask, and quite thankfully, we didn't manage it entirely before the concept of environmental conservation and the continued dependence of man on the wild food web became mainstream about the turn of the 20th Century with the development of groups like the SPCA and Sierra Club. The UN CITES treaty and member nations' implementations of it such as the US Endangered Species Act have stemmed the tide on systematic hunting, but there's still more of it than we want there to be, and prior to these efforts we made quite a go of it.
So, back in your world, a large herbivore that is poisonous to eat by a mechanism not destroyed by cooking (at least not without destroying the meat itself), that is not domesticable in any meaningful way as a working or fur-bearing animal, and that is in any way threatening to humans even on an "I'm standing my ground to protect my family group" basis, is going to find itself on a shortlist for extermination in any time period prior to about the early 20th Century of the real world. Anything that can tolerate these herbivores' toxin and so preys on them for food is probably a threat to us and our domestic livestock as well, so depriving them of a primary food source and putting pressure on the predators' numbers is just another benefit to having the toxic herbivores gone. Even in more enlightened times, anyone saying the toxin could be the cure for cancer is likely to be shouted down; if the species is preserved at all it would be purely for the sake of conserving a diversity of wildlife.
