IMO suicide by ingesting something is already a delicate dance -- you have to follow a regimen, right dosage, your body has to react as expected to the regimen etc etc. Despite following everything correctly, people still fail. Given this, ideally, it's best not to introduce additional uncertainties IMO.
We know for sure that these chemicals work only until their marked expiration date. After that, the guarantee from the manufacturer is no longer there and so you're on your own -- maybe they still work, maybe their potency is reduced, maybe they go bad and cause non-lethal adverse reactions (potentially painful) etc etc. At least in case of medicines (which have expired), all these 3 scenarios have happened to people.
Besides, there are also other factors that determine longevity of the chemical -- how well was it manufactured, whether you stored it in the right container and under right environmental conditions etc etc.