This is somewhat survivor bias though, because all the dead that didn’t survive their youth are actually dead.

In wealthy countries these would-be-dead people walk amongst us.

One of the things they have that people in developed economies generally don't is a 50% infant mortality rate.

The ones that don't achieve it through access to very unnatural artifacts such as vaccines that are quite likely to have been made using ultramodern technologies such as genetic modification.

Or, I've got quite a few friends who have various congenital conditions that mean that they absolutely would not have survived in a society with a more "natural" foodway. With the modern food supply chain, though, they're doing just fine. Unnatural things you get in some ultraprocessed foods, such as vitamin fortification, mean they can even do it without having to worry about developing comorbid chronic ailments due to malnutrition.

That is a survival bias. Ironically if you want signs of good health practice look for unhealthy people - it means that they can survive vs the unhealthy just dying.

A really good example of this was the paper that kicked off the whole "omega-3 fatty acids for heart health" thing. It ultimately got retracted.

The gist of the paper was that they observed that Inuit communities have really low rates of heart disease, and hypothesized that it could be because their traditional diet is very high in omega-3 fatty acids. The problem is, they don't actually have low rates of heart disease. They just have low rates of heart disease diagnosis, because they also have limited access to health care.