The Klingon/Federation war
Prior to Star Trek: Discovery's first season, we had never heard of the Klingon vs. Federation war. We had never heard of the Klingons occupying 20% of Federation space. We had never heard of this war. What's the first thing that fans are introduced to when the show premiered? War!
Now, we know the two sides were at odds for years, we also know they had small skirmishes during their time sharing space. Ranging from the first Klingogn arriving on Earth in Enterprise, all the way to the two sides teaming up against the Dominion.
Yet, during those years, all those instances, we never once heard talk of a war that spanned years and featured untold amounts of death and territory change. You'd think that with the likes of Gowran and Martok roaming around someone, anyone would have brought it up as an insult.
They didn't though. No one did. The writers wanted to shoehorn an epic battle into the past, all because they were once again trying to profit off of nostalgia, but deeming that nostalgia not good enough in the form it was in. So instead they changed things up to fit what they wanted Star Trek to be.