Not really. CS is banned by a technicality. An irritant gas is hard to distinguish from a lethal one and easily swapped if chemical delivery systems are already present on the battlefield.

CS is quite useless against a conventional force because all modern armies are equipped with protective masks in case the other side does use chemical or biological weapons.

If you think that tear gas isn't being used in foreign countries, then you haven't watched television news in the last 40 years.