Someone studies one group of people at one (modern) time and place -- a group of people who probably don't even exclusively "forage" for their food and other economic activity now as encountered -- and then they, or their readers, assume that what they found is somehow typical of everyone they assign to the category "hunter gatherer", and further assume what people at what other times and places are "correctly" assigned to that category too.
https://apps.worldagroforestry.org/downloads/Publications/PD...
You don't even need to go that far - my Italian grandma who was born in a farming village in the early 1900s, knew dozens of herbs, how you would go about finding them, and how could you use them for this or that other purpose. You can't find any of those herbs in a modern supermarket except maybe for a couple of them.
I feel like we've lost some of our food cultures at some point and we're only starting to realize what we're missing.
* meat: my family is from a village near Rome, Roman traditional cuisine is famously full of recipes that use all sorts of weird meat cuts (pork jowl, internal organs of various animals...) - you won't find most of those in an average supermarket. When it comes to meat, the problem is not just cuts, but also breeds: I live in Australia and it has been extremely hard to find chicken which is not Cornish Cross. In Sydney, you can find heirloom breeds chicken at only a handful of speciality butchers, and prices are very high.
* grains: this is a well known problem, most grain-based products are based on just a handful of varietals. I have to say that the situation is starting to improve: recently, in some upmarket stores, it has become possible again to get products based on some heirloom grains, however they are pricey (example: Pasta Monongrano made from Matt or Kamut flour)
* fish: it is a known problem that fishing boats are forced to throw away certain catch, because certain fish varieties are not "marketable"
the list could continue...
There's probably still an effect in there, but it seems more likely that it's mainly antibiotics that are killing off gut microbiota variety, with an argument to be made for a role of highly processed foods.
Of course I'm sure it's highly dependent on season and region.