I think you're misunderstanding the claim. It's name a healthy diet. A single one. A diet which followed to the letter for the time span its prescribed will not cause any health problems traceable back to it. Make whatever assumptions about the hypothetical person you need and assume that they follow the diet for only as long as those assumptions hold.
There is no "healthy diet" that is universally applicable. Go back and look at the list of different possible restrictions in the parent post.
Then add the fact that many people are allergic to all kinds of different things.
Then add to that the fact that people of different ethnic backgrounds are adapted to do better with different balances of nutrients.
Any "hypothetical person" such as you describe would, fundamentally and inevitably, "normalize" one specific type of person and "other" everyone else. Claiming that "this is a healthy diet" in such a way would be grossly irresponsible, even if you then went on to list the features of a person it is "healthy" for.
Finally, even if you wanted to make such a description, the whole point of this thread is that we do not have a scientifically-rigorous-enough idea of what it would be to be able to do so with a high degree of certainty, even for a clearly-defined set of parameters for who would be eating it.
I didn’t ask for it to be universally applicable, or even broadly applicable. Or even applicable to more than the person the question imagines. I asked if we have enough information to know one single example of a healthy diet for one imaginary person who you are allowed to assume has no complicating factors.
Because if we can’t answer this much much simpler problem then we have no hope of giving nutritional advice to a population that is as varied as you describe.
