The use of a wide range of gas agents for different purposes in combat.
Nerve agents are useful for capturing valuable capital assets such as ships intact instead of sinking them. What if the Japanese located one of the American aircraft carriers at the time of Pearl Harbor and instead of attempting to sink it, sent planes with commandos on gliders that detached from Kates and dropped sarin or cyclosarin bombs that cleared the decks before the commandos parachuted in. The commandos then drop more cyclosarin down the air shafts and, wearing proper protection, go down into the ship, killing the crewmen who are still alive. In this way, they have another aircraft carrier and begin to overcome their disadvantage of a lack of capital ships. They can continue this strategy agianst ships in Pearl Harbor and British ships at Singapore, as the Germans can, British ships in and around Scapa Flow and the North Sea and North Atlantic.
The advantage will last until the Wallies and the Russians copy the tactic, which brings piracy on the high seas back in a very ruthless way. And since tanks also need air, a magnetic sarin vaporizer near an air vent can disable a tank in short order.
If the Nazis are engaged in murdering Jews and other civilians ITTL, soman, being quicker, is much more efficient than cyklon-B if it is not in short supply.
Residual organophosphates IOTL that can contaminate an area for months such as VQ and VX were not developed until the early 1950s by the US IOTL and never actually used even though the US did not sign the anti Chemical and Biological warfare convention until the 1970s. But Tabun, Sarin and Soman, if sprayed on surfaces when an army is withdrawing from an area in retreat can contaminate unsuspecting troops entering buildings that have been painted with it, at least for a few days. And if it becomes a priority, residual contaminants might not be difficult to develop.
For dealing with civilians in cities and villages, simply a lot of tear gas can be useful. As American forces used in Vietnam. Not all gas warfare need be lethal. And tear gas can interfere with the combat effectiveness of unprotected troops.
As can hallucinogenic gas agents such as BZ that the USAMRIID and CIA experimented with as part of MKULTRA. But those were not developed IOTL until the 1960s.
Vesicants such as Mustard gas, lewisite, and Chlorine were initially developed by the Germans in World War I as souped up tear gasses, not neccesarily to kill allied troops. They are of limited utility except to burden the enemy with wounded and incapacitated soldiers, some of whom will die, others who will not die, but all of whom must be cared for.
. And that goes for all sides and for civilians.as well, if one is engaged in strategiic bombing against civilians.
Finally, there is incendiary or thermobaric chemical warfare, which all sides did use IOTL to the extent that they could and that all nations have continued to refine as an exception to anti-chemical warfare treaties. Thermobaric agents, be they napalm, coal dust (the Germans experimented with it in WWII), ignited tear gas or ignited natural gas or propane on an ad hoc basis create a flame front and blast effect and airburst that can destroy several city blocks. This is the "blockbuster" bomb that the Allies dropped on German cities. And this is the BLU-158, the "daisy cutter that the US uses today. These are chemical weapons that kill people by incineration of their skin, lungs and other tissues.
Sometimes thermobaric weapons are the only way to destroy enemy bunkers or the people inside them, as the US did Japanese soldiers holed up in caves. with flamethrowers. (Is it more humane to burn someone alive than to poison them with a pesticide that kills them within a minute?) And sometimes, as Arthur Harris and Curtis LeMay did with strategic bombing against German and Japanese cities with white phosphorus incendiary bombs that had thermobaric firestorm effects, thermobaric bombing is used simply to destroy civilian lives in the hopes that it will end the enemy's will to resist.
So it's not just a question of whether chemical weapons are used routinely, but the military use to which chemical weapons are put.