That’s one of the reasons why you never find bleach and ammonia one same shelf or in the same isle. I am not so sure if it makes mustard gas, but whatever it is will take your breath away if these two get mixed.
"Mixing bleach and ammonia can produce toxic gases, including chloramine gas, which can be poisonous if inhaled.6 The reaction between bleach and ammonia is highly exothermic and can potentially cause explosions if the area isn't ventilated.2 The primary toxic chemical formed by this reaction is chloramine vapor, which has the potential to form hydrazine, which can cause respiratory irritation, edema, headache, nausea, and seizures.4 Mixing bleach with some household cleaners can generate chlorine gas, and mixing bleach with ammonia generates toxic chloramine vapor.0 However, death from mixing bleach and ammonia is very rare.1 It is important to always read the product label before using a cleaning product."
https://www.healthline.com/health/bleach-and-ammonia
Makes chloramine gas, not mustard gas which is a blister agent.