In The Force Awakens, there is a scene in which the main characters

are planning on poisoning a group of storm troopers because Fin states that the armor does not filter out poisonous gas.

However, according to this answer, http://scifi.stackexchange.com/a/14458/29220 stormtrooper armor contains

A vacuum suit with a 20 minute air reserve, as well as possessing air filters for providing protection against airborne pathogens or gas attacks when air was available

So what happened to the armor in the last 20-30 years?