They are allowed only if the question itself does not focus on the nutritional content of the supplement, and the question is related to preparation of food.
That is, ask "if the nutritional supplement were taken out of this question, would it still be a question?" If the answer is "no", it's probably off-topic.
So, as an example, "I'm adding spinach extract to the vegan dishes to increase iron content, how do I disguise the taste?" would be on-topic and allowed. But "Should I add spinach extract to a vegan diet?" would not be, because it is a request for nutritional advice.
I'll also point out that the first two questions you link are not about dietary supplements. The third, which is, is from 2010 and probably would closed on the site today -- we've gotten stricter about enforcing on/off topic, mostly because we got flooded with health advice questions. See comments on that question that it ought to be closed.