Oxidation of fat makes food smell bad and causes toxicity. Preservatives prevent this. Vitamin E, for example, is a preservative. It prevents oxidation of fat in the body and plays a similar role in pet foods. Not to our knowledge.
Most controversy surrounds the synthetic preservative ethoxyquin.
Nevertheless, ethoxyquin has undergone long-term multigenerational trials in dogs but there is little information in cats. If you are worried about this, however, feed a canned diet because they are not exposed to air after manufacture, are less likely to oxidize and contain less preservative. Adjust food intake to maintain optimum body weight and condition. Do not feed too much. The slim-model is best.