I have an interesting adjacent question:
Those DMs who run carefully curated worlds: Do you go through the Monster Manual (or any other monster book) and categorically decry "this monster does not exist in my world"? Not, "I haven't decided" or any other non-committal answer, I mean "this does/doesn't categorically exist." Because I think the majority of DMs wouldn't go through the MM on Day One and whittle down their options. A particular monster might not exist in the area the PCs are in currently (or conversely, the "known world") but I don't think many DMs never-say-never. Because they don't want to limit their options when it comes to telling stories/using new threats. And most monsters exist in a Shrodinger's Box of "they don't exist until they show up in game"
Yet I imagine the same DMs have few qualms doing the same to PC options. To put it another way, if Dragonborn were just a monster in the Monster Manual, I imagine most DMs would not consider banning them outright on day one. However, being the PHB, they get a Day One pass or ban. The Players live in a world of black and white hard-defined options, the DM gets to play around in the gray "maybe this exists, I haven't decided yet."