There never was anything called Zyclon-D so this is not a thing. There was Zyklon (later renamed Zyklon A) and Zyklon B, the latter was renamed Cyanosil somewhere in the '70s and might still be produced. The active ingredient for all was hydrogen cyanide which is still used for fumigation purposes. It was also used in the USA for execution purposes and might soon be used for that purpose again, at least in Arizona [1].

[1] https://apnews.com/article/arizona-executions-business-healt...