Suggestion R3 We can postulate that, since there is no way that a straight answer to the question will satisfy food safety guidelines (to my knowledge, the FDA has never approved a preservative-based method for home use), the question is inherently opinion-based, since it uses a definition of "safe" that is based on a personal judgement. In that case, these questions would have to be closed, not because they are out of scope, but because they are opinion-based.
I am adding this option because I believe it was the line of thinking in the post that prompted the question. Advantage: It makes it clear to users that we do not want answers based on personal conviction, like "Just throw in 0.5% of magiconium wondrate and you're good". The disadvantages are: a lot of frustration to the askers, and missing the opportunity to explain to them why their mental model of food preservation is wrong.