Forcing "purity" (interesting term for the topic, isn't it?) in an RPG is a slippery slope.
Sure, Genre-wise you don't want Luke Skywalker using 2022 tiktok meme examples in RP (he's supposed to be from a galaxy far away in a long time ago), but there's a pretty big line between "Creative preference" and "tyrannical GM nonsense". In the case of BofD and these two mentioned in the topic, their angle is that they view themselves to be an authority on the setting and they're literally inviting people to play content they view themselves to be owners of. So I don't get why people even bother to play that game, because ultimately it comes down to playing "house" with a couple of kids in their backyard and they'll make you leave if you're "not doing it right".
Why bother trying to be creative or create ANYTHING in an environment with someone who will tell everyone what their characters are, what they think, what they would or wouldn't do, etc?
These people are all "No" and not "Yes, and". They're not even "maybe, so" or "No, but". At the risk of sounding snide, they're literally using someone else's literary work and reputation to build themselves as an authority on the content and gatekeeping the shit out of it...and one of their preferred gates to keep is that BIPOC aren't in the ruling caste, make racist jokes, and consider black characters in the setting to be a matter of "woke cancel culture".
So, constructively, this should all be a lesson on do's and don'ts when it comes to understanding a few key things.
if you don't want people to try to create their own things and having a stake in it, you should be solo-writing it and not running it as an RPG where you falsely promote the concept of logging in to have agency over...well...anything.
Yes, it's true that players/fans blur the concept behind a character's skin color as being indicative of support for ACTUAL people with that skin color. Is it a minefield? Yes. Is it 100% about the content? No. It's somewhat shades of politics, social issues, etc blurring it's way into content. Still, it's best to understand this and take at least a "maybe, but" approach.
You could create a setting where 1000 all-white families boarded a spaceship and colonized a planet, and after 200 years there are 10,000 people who have ONLY mated with people on that initial all-white spaceship, and people will STILL be like "...can I play a black character?" and your answer should always, ALWAYS be to question why the fuck you decided creatively that everyone in that ship was white to begin with. What's up with that?