Though this is my anecdotal experience, I've had an interesting journey related to food sensitivities and realizing how much certain foods actually caused physical pain when eating them once I had removed them and allowed my body to become de-conditioned to that level of pain/stress; conditioning is the action of not noticing, eliminating it from your awareness, however from my experience that shows that that doesn't mean something you don't notice isn't harming you or causing you problem. I had done an Igg food sensitivity test (inflammatory marker diagnostic; just a few drops of blood on a card that would get sent in) to test for ~200 foods. I removed the foods that showed up and I felt better. Weeks later multiple times with different foods I would eat one of the foods on the list and I was amazed at how clearly I could feel, experience the physical pain - some severe - that I would experience from now eating it.
This brings me to another point I wanted to make. You said the term "healthy" shouldn't be used, I'd posit that a caveat is necessary to avoid generalization - generalization which is the actual culprit: "may not be healthy for you specifically." It's very naive to ignore that though most of us have a head, two arms, two legs, etc. that our inner workings aren't complex and diverse - including our sensitivity level and tolerances for different types of stressors. We can know however inflammation in the body can lead to Alzheimers-Dementia, and that sugars carbohydrates, and certain other foods like dairy are highly inflammatory (for everyone) - whether that causes enough dysfunction for you to eliminate them from your diet or not is one question. Another question is perhaps a person is using these foods to self-medicate: inflammation has a depressant effect on the nervous system, and so they could be eating certain foods - perhaps dependant on them - to regulate their mood, stress, etc. This may work for some people their whole lives, it may also lead to dis-ease progression over decades.
And to reference your Paracelsus quote, it's not solely the dosage but a dosage that acts poisonous will differ between people. Some might be okay for some people, any tiny amount may be problematic for some people. The vague and arguably dismissive rhetoric of "everything in moderation" in response to when people reply in regards to people's comments on eating food they know is unhealthy for them and makes them feel sick. The dismissive behaviour of of parents for when their children not liking certain food and not wanting to eat it, they force or incentivize them to continue to eat food that harms them with a reward like dessert that further harms them is also problematic and I believe the starting point for many people who then struggle later in life in various ways. Eventually the child may become conditioned to not feeling the harm of the food, perhaps with the added aid of the numbing effect of the inflammation (the depressant effect on the nervous system), and become disconnected from a foundational cause or source of future dis-ease progression related symptoms. Parts of this pattern were certainly part of my journey.