Settings used to be more fluid, I've noticed. Back in the TSR days, if a new supplement or Dragon magazine article came out and said "hey, here's this new class or race" and provided lore for what campaign settings it could fit in, an individual DM might not allow it, but I don't recall anyone saying "x doesn't belong in y setting". Maybe this was just because the internet wasn't quite a thing yet, maybe it was because with Spelljammer and Planescape, TSR's worlds were more cosmopolitan, with it not being a big deal if a Tinker Gnome from Krynn hitched a ride on a spelljammer and landed in Waterdeep or a Bariaur stepped into the wrong portal and found himself in the Free City of Greyhawk.
By the time WotC took over stewardship of the IP, the existence of Aasimar and Tieflings wasn't contested, and we got a ton of new things too, like Genasi, and several books devoted to dragonkind which gave us "draconic" PC options, (like say, Silverbrow Humans).
But during the 4e era, I started to see a growing rise in people upset about not only Dragonborn and Tieflings in the PHB*, but the idea that (gasp) someone might dare play a Warforged in Tethyr, as if the idea of a
sentient golem person in the Forgotten Realms (the setting where new things pop up all the time, from Saurials to entire friggin' continents- lookin' at you Maztica and Zakhara). In fact, it doesn't take a lot of explanation as to why such a character could exist. If someone didn't want all the Warforged and Eberron lore cropping up in their campaign, you could easily provide new lore, or use the racial package to represent some other constructed creature (like a Golmoid, from Dragon #317).
*as if Gnomes didn't themselves appear as a PC option for the first time in the 1978 PHB.
Don't get me wrong, if a DM doesn't want someone to play a Mul in the Forgotten Realms (half-dwarves already existing in said setting) or a Tortle on Krynn (despite the setting already having some crazy races, likely because of the Graygem, like Walrus-men), that's their prerogative, of course. A lot of DM's don't like their PC's being a menagerie of oddballs (because of course, there was never anything unprecedented about a Dwarf, an Elf, a Human, and four Halflings going on a quest together), and that's fine- but the irritation some people have with other options
existing, even being in the sacred PHB, with the game developers saying "hey, there's a place for these guys in your favorite settings" as if there's never been a precedent for such things throughout the game's history, always struck me as a bit strange.
Especially since I've never heard the same complaints about, say, new monsters being added to the game, often with very little real lore at all beyond "so here's this crazy monster". Is there a fundamental difference from adding Mudmen to your campaign because they appear in Beyond the Crystal Cave and adding Valley Elves to your campaign because they appear in Vale of the Mage, despite not having been even mentioned anywhere previously?
