A lot of this thread seems to be imagining people and situations to get mad at, because surely session 0 and talking to people in good faith would solve all of these problems 99% of the time?
But like…if a player REALLY insisted on being a tortle, and my setting REALLY didn’t have tortles, I would just let them be a tortle and not interact with that part of their character. There wouldn’t be any tortle npcs, no npc would ever remark on their tortleness. If they ever did something tortlely, I would smile and nod politely, then move swiftly on. If they don’t want to fully engage with the setting, I’m not obligated to fully engage with their fantasy. The game can still go on.
The vibe I get from the folks who really want to be a tortle (or whatever) no matter the setting is the same vibe I get from people who go to a chinese buffet and only eat the pizza. And that is totally fine. We are here for different purposes, but we can still hang out and have fun.
Now if all the players want to be tortles / some other off-setting option, then that’s different. It’s clearly not a chinese night, this is a pizza night. So I will make a delicious pizza for us all to enjoy (by which I mean I would either change or make a new setting to fit the vibe).
The notion that GM’s shouldn’t be too precious about their worldbuilding is very fair, but I think it applies to players too. Why do you insist on being a tortle? Maybe you would be better served commissioning some art of your OC, or getting into Second Life?
D&D, and especially 5e, is the kleenex of rpgs. Most people aren’t playing it for its breadth of options, they are playing it because it's the default. I feel like the expectation is that you are going to make the game your own, pruning some options and adding some others that better fit your table. Just talk to people about it.