It wasn't closed as "irrelevant", it was closed as off-topic. Not everything to do with food and cooking is within the scope of the site. The second section of the faq lists some common categories of off-topic questions, including "General health and diet issue (e.g. 'Is cauliflower healthy?')".
I'm not sure why you want to compare this to the soda question; while that question does have issues, it's about kitchen equipment, which is a perfectly acceptable topic
Chemistry of food and cooking is of course on-topic. A variation of your first example question is fine: "do claypots leach minerals into food". Whether those minerals are good or not is a health/nutrition question, though, and that's off-topic. Your second example about leaching BPA is also fine.
But questions about health and nutrition have always been off-topic here. We're not a health site, and we're not going to start giving medical or nutritional advice. We can answer food safety questions like "is meat safe to eat if it's been left out for 12 hours?", using accepted food safety guidelines (generally from government agencies), but we're not going to get into health questions like "how much meat should I eat?", "is chicken healthier than fish?"