For example, is it ok to ask "Some people say that drinking water after a meal is harmful for us - is that true"?
No, that kind of questions is very clearly off topic.
We are primarily cooks here. Our common interest is food. We can not give medical advice, in fact, many questions of the “Is X healthy?” type are not answerable for medical professionals.
The types of questions we will accept here are:
Questions with a limited scope and measurable, scientifically determinable answers, e.g. “Do apples or fava beans contain more vitamin C?”
Questions about food safety, that is the immediate effect caused by pathogens or methods of food handling, which we will answer according to the official guidelines by institutions like the FDA or similar. Example: “How long can we safely keep cooked potatoes at room temperature?” Note that we have a few generic Q/As that already cover many cases.