Perhaps the diversity lost is the one caused by seasonality. Fruits and vegetables are highly seasonal, some meat and fish are, too.
From reading various articles, apparently there really was greater diversity in vegetables, too. Nowadays people eat tomatoes, salad, onions, perhaps carots, and that's about it but there are many, many more that can be locally grown and that have almost disappeared from many people's diets. If you mention swedes, turnips, cabbage, beetroots, leeks, rhubarb, types of squash, etc. even cauliflowers to many people they won't be able to tell you when was the last time they ate or touched one. The only time people touch a pumpkin is at Halloween and they don't eat them.
Regarding meat, offal has almost completely disappeared, and I suspect seafood is now mostly processed fish from one or two species only.
> swedes, turnips, cabbage, beetroots, leeks, rhubarb, types of squash, etc.
Perhaps this is a U.S. issue. Every single one of these is available at my local Lidl supermarket (~a marginally upscale Aldi for those outside Europe) for much of the year, not to mention other more starkly seasonal things that filter in like peaches, salsify, runner beans, cherimoya (a tropical fruit), and so on. Across Europe I've had no trouble getting all the offal I'd want: head cheese/zult, haggis, black pudding, balkenbrij, hearts, various horrors suspended in aspic-- in supermarkets. Not specialist shops, not the butcher. I'm in rural Ireland now, but the selection was better yet when I lived in Switzerland and NL and also shopped at Lidl.
Those are all available in any US supermarket well. The quantities on the shelf suggest they don't sell well, just enough to be worth having.
I should have mentioned that I am in the UK.
Many of them tend to be available in supermarkets, though it varies by location, but, really it's only older people who buy them.
That said, coming from the continent myself, the UK is below average. For an island the general lack of a variety of seafood is especially stricking.