And, stupidly thinking that I might have invented something (!), I was surprised and humbled to find that the first thing that Sheldon Brown noted in the previously linked article, was that lawyers have made once-flexible forks stiffer on modern bikes to avoid liability claims from riders who break them and hurt themselves, but that more flexible forks (and risks) were more common on vintage bikes. He also notes that rear became stiffer (though due to criterium geometry) too. Longer stems and seatposts are perhaps merely a way of getting that flex back.