It’s an interesting question: What episodes of Star Trek: The Original Series are explicitly in continuity because they were referenced in the Trek movies? Could you state that, say, “A Taste Of Armageddon” definitely happened to our heroes because it was referenced in one of the films? Could you find references to the entire television series if you looked hard enough?
It seemed like an interesting challenge. And since I was going through a bout of insomnia when I first read the question, I decided to find out.