So just by way of example, one mechanical thing that is new in 5.5 that I think would impact the way we write D&D settings differently than how we have done it so far, is the change in how species and background interact mechanically. previously, we did a lot of pastiche Tolkiening -- this race (or subrace) is like this and lives here and so on. 5.5 throws that out in a big way. Those themed locations (your Rivendels and whatever) would no longer be species based, but culturally based. that is a big deal in a setting design.
You could but there's nothing saying that you couldn't before. Having a cultural tendencies but they're just tendencies not dictates of all possible options. It was also effectively added with a splat book, not the 2024 PHB.
I think it matters more for players (and players overemphasizing a +1) than it would for a large population. The normal variance of abilities would make more difference at the individual level than racial adjustments.
On the other hand doing less monocultures is not a bad idea even if it is a common trope in fantasy and sci-fi.