Agent Orange, used in the Vietnam War to expose Vietnamese positions by removing overarching jungle, is particularly well-known. It caused cancer in both Americans and Vietnamese exposed to it. Yet, major ingredient 2, 4-D is actually still very much alive and dangerous - used on crops to where genetically-modified (GMO) corn is being called "Agent Orange corn." Agent Orange Food Disorder, or pesticide disease, is caused by a steady stream of consumed toxic pesticides in general(2).
Zyklon A and B and Agent Orange, in other words, technically didn't die out in history. The fact that such substances were allowed to continue on in various forms begs the question, are they the only such historic killers/poisons out there? Truly, there are tons of others.
Sources:
1. "Zyklon B." Wikipedia, Wikimedia Foundation, Inc., edited 16 March 2019, https://en.wikipedia.org/wiki/Zyklon_B.
2. Wells, S. D. "What is A.O.F.D., Agent Orange Food Disorder, and how many Americans have it right now?" Natural News, Truth Publishing International, LTD., 13 October 2012, https://www.naturalnews.com/037534_Agent_Orange_Food_Disorder_AOFD_chemical_exposure.html