I think the biggest restriction on the use of chemical weapons would be that they aren't really in military terms all that useful. I think if they were "off the table" during conflicts like, say, the OTL Soviet-German war there was probably a reason other than international law.
If you look at the OTL use of chemical weapons- the Italian invasion of Ethiopia, World War One, the Sino-Japanese war, and the Iran-Iraq war, you can see that extensive use of poison gas occurs when powers are grasping around for some, any sort of military advantage or edge against superior numbers/a quagmire AND are free from retaliation (Iraq, Italy, Japan) or have exhausted all other options to break a stalemate AND are short of conventional munitions (World War One).
If chemical weapons become a "normal" part of warfare they'll have a niche role in especially nasty sieges and colonial wars, but I don't think they'd wind up in heavy use in conventional peer-to-peer conflicts, because they debilitate the user about as much as the target, and are paradoxically easy and really really inconvenient to defend against.