Heirloom varieties date from before World War II. They are those few old varieties which gardeners choose to maintain because they are good (Let’s not get too starry-eyed about old varieties; most were lost because they weren’t that good). Heirloom varieties seem particularly common amongst plants valued as food or for their beautiful and complex scent.
Many heirloom varieties result from purposeful crossings by gardeners, market gardeners and smallholders, fewer are the result of chance crosses. Most date from less than a hundred years or so ago, a very few may be medieval or even pre-history. They were selected by being observed to fit our purposes (empirically) rather than by scientific measurement.
Established varieties are modern varieties (developed post-war) and are similarly being retained because they have maintained a good reputation amongst gardeners over at least two or three decades.
Gardeners, myself included, often grow heirloom or established varieties; there are small seed companies thriving on the sales of only these varieties. They often appear to suit our needs better than ‘new-fangled’ ones.
This is partly because gardening practices haven’t changed much. We still use mostly our manual labour to cultivate the plants and we pick the fruits of this by hand too! The gardeners who mostly bred them also ate the yield so they valued how good they tasted, as we still do! And heirloom varieties had to grow more-or-less unprotected by modern pesticides – and most gardeners value that they can grow their own produce more-or-less organically.
Yet our heirloom, varieties are all ‘old’, bred in a time when stable manure was widely available and used, the climate was cooler and the CO2 (the main food of plants!) in the air was half its current amount. Heirloom varieties which are not native are also popular, for example, here in the UK we grow Ratte potatoes from France and Blauhilde climbing French beans from Germany so not all heirloom varieties were even bred for local conditions!
So our retention of them is also somewhat odd.