The lumber they used for structural members (huge, 8x10ish boards, very long, all around where the foundation met the house; joists; et c) or for other parts that were never meant to be seen, like floor underlayment (which was a bunch of ordinary boards laid diagonally, so they wouldn't match up with wood floor boards laid atop it—not plywood) would probably have gone to veneer these days. Knot-free, perfect stuff. Much of it's likely nearly impossible to get these days at any price, in the same size and quality, short of salvage from houses like that. I never got much of a look in the walls, but it was 2 stories plus a full-height floor for the attic (need that for temp control, in the days before air conditioning) and was "balloon" construction, which means they used a whole bunch of boards 2-3 stories high to frame the exterior walls, which, again, probably nearly impossible to find these days, straight and strong enough to do such a thing.
I notice that working-class houses of the 60s-80s (and a bit into the 90s) usually had solid wood trim and doors, too, typically throughout. Those are now luxury-tier, and even rather expensive houses will use particle board (basically cardboard—sawdust and glue) or pieced-together pine that must be finished with paint, and hollow doors, often without even finish-grade veneer (so they have to be painted, can't stain and varnish them). Expensive houses where the person building it cared a little more (or the buyer had input) may put solid wood in certain public areas, but e.g. bedrooms? Hardly ever, now, at least in houses built for/by people who work for a living. So the process of lumber-worsening may be ongoing.
I am not positive, but increasing temps and lighter winters, more CO2 in the atmosphere, probably also increase the growth per season even in a forest that hasn’t been logged, if there are any.
They are pretty nice!
It wasn't red oak?