[jillesvangurp]

It's probably more because of the horrific effects it has mainly on non-military targets and the fact that people tend to be held accountable afterwards. As happened to e.g. Chemical Ali, a general in the Iraqi regime famous for his use of chemical weapons on civilians, who was hanged in 2010 for his actions. Likewise the mere hint of such weapons being used in Syria, caused them to be isolated further.

So, most modern military would be keenly aware that the use of such weapons might have long term consequences for them personally and hands the moral high ground to their opponent almost automatically almost immediately. This has mostly negative consequences short term and it's probably counter productive. If that was not the case, we'd be seeing those weapons used in e.g. the Ukraine right now by the Russians. And they seem to have no reservations about targeting civilians otherwise. But it's hard to spin gassing civilians as heroic or patriotic. Even in Russia. Kind of destroys their de-nazificaton pitch.

Even the Nazis did not go there during WWII. Aside of course from committing genocide as part of the holocaust. They did actually develop chemical weapons but they never deployed them on the battle field: https://www.history.com/news/the-nazis-developed-sarin-gas-b...