I’ve never sat down and looked it over, but it certainly feels like every TOS episode has been followed up in one form or another at this point.
I think there are a few, if we go by Laura's definition that it has to be a direct continuation of a specific plot or character thread from the episode, rather than just the reuse of a concept. Let's see...
"The Enemy Within" wasn't something that really could be followed up directly. The closest thing is the fan story "Ni Var" reprinted in edited form in Star Trek: The New Voyages, in which a scientist developed a variation on the transporter-splitting effect to split Spock into human and Vulcan halves. That's maybe a borderline case.
"The Man Trap," as discussed, has had no major followup, just passing allusions. "The Naked Time" had a canonical sequel but no prose or comics sequels I can think of. I'm not sure if "Dagger of the Mind" has had a direct followup, though I think there was a comics story involving a modification of the neural neutralizer.
"The Galileo Seven" is a borderline case; Boma reappeared in Dreadnought!, IIRC, and there was a reference to the events of TG7, but it's a peripheral link. Beyond passing references, I don't think we've had significant followup to "Catspaw," "The Immunity Syndrome," "Patterns of Force" (unless there's a John Gill prequel story I'm forgetting), "Spectre of the Gun," "Spock's Brain," "Plato's Stepchildren" (I've always wanted to bring back Alexander but haven't had the opportunity), "Wink of an Eye," or "Turnabout Intruder."
You could probably add a fair number of TAS episodes, but I'm tired.
With Picard season 3 now complete, there’s been a lot of discussion about it being too much fan service, etc. I think Star Trek on TV/film has been much more reserved when it comes to follow ups or overindulging in nostalgia versus Trek literature and comics. As a reader of the post Nemesis books, there’s initially a reaction of ‘oh really, the Borg, Picard and Seven again?’ but for the overwhelming majority of the audience these are concepts and characters that haven’t really been addressed in decades (Picard season two notwithstanding).
I think there's a fundamental difference there, though. Tie-in novels' express purpose is to tie in, to support, to follow the lead of canon and elaborate on what it established. We're supposed to be derivative. Canon's job is to take the lead, to set the course, to introduce the new ideas that we tie-in authors elaborate on. If the canon ends up just elaborating on its own past instead of establishing new ideas, that's a sign of stagnation. A franchise that becomes nothing more than fanfic for its own past is no longer moving forward. Which is why I think we need less Trek like Picard seasons 2-3 and more like Discovery and Prodigy. And Lower Decks and Strange New Worlds, which do rely heavily on nostalgia but manage to balance it with original characters and stories, as well as proving that episodic storytelling is still worthwhile.