This isn't exactly something that was "lost" but more something I'd like to see preserved...
I want stone-crafting, gold-mining, mountain (or hill!)-living, ale-swigging dwarves.
I want forest-dwelling, magic-making, bow-wielding, wine-swigging elves.
I want hill-farming, pipe-smoking, sling-...um...slinging, mead-swigging (hm. Am I seeing a "swigging" pattern here?) halflings...WITH HAIRY FEET! (call the skinny boot-wearers whatever you must and be done with it. But for the love of the gods burn the "flat-headed, corn-rowed, river-gypsies"!)
Yes, yes. I know. We are all soooo "over" that. That was 40 years ago and we're infinitely more creative now. I know. We are! But you have to start
somewhere. And for the core/basic books there needs to be a baseline.
You can have all of the sea-faring gnomes and planet-jumping elves, swamp dwarves and arctic winged halflings you like. Stat out 20 "sub-races" for each if you want. But they don't need to be in the core/main books.
The basic/core books can't (and don't need to) give you stats and abilities for every possible permutation.
There's a very real limit/ratio to page count and printing/product price...not to mention the risk (or inevitability) of "bloat".
The initial releases shouldn't say 'Well, dwarves are 4 feet tall, have beards, and can't get knocked down. The rest is up to you."
But nor should they say "Here's six pages of various dwarf cultures, 10 pages of elves, 4 pages of gnomes and, oh yeah, there's halflings too (but we won't make a big deal outta them so as not to ruffle the Tolkien estate)."
Stick to the classics...flesh them out to make them feel like "real" peoples and cultures. Leave the rest to individual groups/DMs imaginations and tools for building whatever they like of their own devising.
<breathe. huff. breathe. exhale> Ok. I'm back now. Fin rant.
So yeah, fluff the races...please.