Heirloom vegetables grow more slowly and spoil much more quickly than modern crops. This is because modern crops have less nutritional value - https://jeroenvanbaar.substack.com/p/data-dispatch-4-the-falling-nutritional
That article recommends eating a better diet. Sure. Seems a bit idealist. Here's some more actionable advice: everyone should take a multivitamin and magnesium glycinate. If you live somewhere that gets a winter take vitamin D too.
I'm speaking from experience here, I used to get sick every winter and my skin would get so dry it would crack and bleed. Take your vitamins.
If food is less nutritionally dense but equally filling, on a population level malnutrition will become more common. The conclusion is that we should all eat more vegetables, I'm not sure what you're talking about with the "fresh produce is bad" thing. This has nothing to do with GMOs or depleted land, it's the result of markets making our food. Produce from local farmers markets is more good than supermarket produce but it's all good.
Here's a study showing people that supplement have less deficiencies than people that don't: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5579642/
Also vitamin D is it's own thing, deficiencies in that are caused by office jobs not diet. It's a hormone. Almost everyone should take it daily.