Coal gas, as in “artificial gas” (as opposed to natural gas) is no longer used in residential environments basically anywhere. It’s literally 50% carbon monoxide and much more toxic than natural gas which is what modern ovens use.

It is impossible to kill yourself with coal gas if you don’t have access to it. People can and do still kill themselves with carbon monoxide by leaving their cars on in a closed space, but that takes more time and effort and people have time to contemplate their decision and change their mind. This is a good thing.

Also, I’m not sure I understand your point about it being a trend. The data shows that total suicides dropped, not just suicide by oven.