Most plastic foils used in the food industry use, besides the main material (whether LDPE or other) a sophisticated sandwich of barrier layers aimed at blocking... oxygen. I have wine bladders set aside for storing developer, and they have a metallic appearance, probably a submicron thick aluminium layer. The oxygen barrier may also be a special polymer, also generally submicron thickness. I would rely on the 100's of man-years of R&D invested by the food and polymer industries. If in doubt, run control strips at regular intervals.Many plastics allow oxygen atoms to pass through, ruining the developer sooner.