[EarlKing]

It's not even that. The purpose of mustard gas, for example, is principally to force unmasked troops to vacate the trenches. As a rule we don't do much trench warfare anymore. (Yes, yes, I know... Ukraine enters the chat) With most warfare over the last century having been wars of maneuver, urban conflict, aerial bombardment, or artillery duels... there's just not much use for chemical weapons. They're only useful for clearing people out of confined spaces... hence why tear gas continues to be used for crowd control in cities.

[MichaelCollins]

Mustard isn't like tear gas; it's persistent. It's not really a gas at all; it's actually a mist of liquid droplets. The mist then condenses on everything in the area and contaminates it for weeks. Anybody who comes into contact with it without protective gear (more than a gas mask) will suffer debilitating injuries. If you get it on your skin, your skin will fall off. This makes mustard generally useful for area denial, particularly against opponents that aren't ready to suit up and decontaminate everything. Drop a lot of mustard on a supply depot, command post, etc, and all of that equipment now needs to be decontaminated before it can be used again.