I agree, with three active shows and potentially two more on the way, it's time to again have a place where people can get some answers from a self-appointed expert.
If Trek were to be "cancelled" for cultural offenses, the Next-Gen episode "Code of Honor" comes to mind as the most egregious, but TOS had its share, of which the casting of Khan is but a minor entry.
As for the question of timeline divergence--a question is, how strictly do you want to take that? For example, a Federation ship definitely did not land at Roswell with a Ferengi on board. If, perhaps, we want to make allowances for the Star Trek "version" of familiar events, there is an excellent "Eugenics Wars" series by Greg Cox which took place in the 1990s, and could plausibly, if you squint at it, occur not significantly differently than what most people believed was going on then.
So I might reframe the question as what was the first divergent event in Trek's history, that definitely would have been known to all in the real world. I think the Millennium Gate episode of Voyager is a good candidate for that. I am however interested in what you have come up with.
"If fighting is sure to result in victory, then you must fight! Sun Tzu said that, and I'd say he knows a little bit more about fighting than you do, pal, because he invented it, and then he perfected it so that no living man could best him in the ring of honor. Then, he used his fight money to buy two of every animal on earth, and then he herded them onto a boat, and then he beat the crap out of every single one. And from that day forward any time a bunch of animals are together in one place it's called a zoo! (Beat) Unless it's a farm!"