Funny enough, the Lieber Code adopted by the Union during the Civil War, dealing with the rules of warfare, DOES actually prohibit the use of poison, so this idea would have been illegal even at the time.
The US Army has an awful lot of dubs during the civil war. A prohibition on chemical warfare wasn’t something I anticipated but I appreciate another W.
This got me thinking about how long have we been putting chlorine in pools. This random website says that it was "first suggested" in 1894 (without any citation, so take that with a salt water pool's worth of salt).
So shells filled with chlorine gas would not smell like a swimming pool during the US Civil War. Because a swimming pool probably smelled like stagnant water and piss.
To be fair pools still smell like stagnant water and piss... And chlorine. Funny story. If you smell the key pool chlorine smell like someone put way too much in. Guess what it's because the chlorine is reacting with someones mungy piss .