Just because we've always done something one way doesn't mean we have to keep doing it that way. It's pretty clear that D&D going forward is going to have a different feel. The nostalgic editions still exist for those who want them.A real history, culture, and mythology ninja would be pretty boring....basically a thief with disguise, lots of improvised weapons taken from farmers, and no sneak attack.
A totally divorced from history, culture, and mythology ninja would be not called a ninja.
D&D has always been a wazoo mish-mash of every single thing that could be taken from history and made into something completely fictional but fun to play as a character, or kill with a character.
I wouldn't run it out of concern of cultural appropriation and wouldn't want to squash the stereotypical behaviors from my players that would result from such a campaign at my table. But sure, let's see it for those who want it.