I believe you’re referring to this article which makes the point https://acoup.blog/2020/03/20/collections-why-dont-we-use-chemical-weapons-anymore/ https://aco

I believe you’re referring to this article which makes the point https://acoup.blog/2020/03/20/collections-why-dont-we-use-chemical-weapons-anymore/ https://acoup.blog/2020/03/20/collections-why-dont-we-use-ch..., though with a slightly different conclusion: it says that chemical weapons are useless for a top tier modern army, because top tier modern opponents can easily protect against them, and other opponents are better targeted using conventional means. But for armies that are not top-tier modern armies, they still make sense and are still in use.