Since the USS discovery is discovering technology and species that is shouldnt have at this point in the timeline
...It really is chiefly an issue that the current heroes
should discover all that stuff, it being out there waiting for them, and we need to invent excuses why all this wasn't already known by the time of Kirk.
I mean, it doesn't take much to get to the Mirror Universe. How many times has this happened, and been classified, in the past century of human transporter operations? In the past thousand years of transporter operations by certain other UFP members? The Gorn (or insert any species name, really!) live nearby; who last stumbled onto them, and why was the encounter classified?
On the other hand, nothing about the tribbles is new in TOS: all the players there know they exist - they state as much. How likely is it that our TOS heroes are the first foolish enough to feed them after midnight? Not very. But Cyrano Jones might be the first one idiotic enough to sell non-neutered tribbles.
As for excuses for why classifying need not be required in every instance:
A compound which gives people powerful telekinetic powers and would be really useful when combating Klingons/Jem'Hadar/random superhuman alien of the week.
Kironide was well known to the UFP for its "great power" before the episode already, but its effects on people were a surprise. Supposedly, then, it has no superpowers except on planet Platonius. Good luck luring Klingons, Jem'Hadar or RSAs there!
An uninhabited planet within federation space with a sentient device on which can transport people to any place or time.
...And thus is rather capable of defending/hiding itself, via manipulating time if need be. Just because it can doesn't mean it will. Or that it won't, even if you ask it pretty please not to.
An ancient alien civilisation on friendly terms with the federation who have vast starships beyond anything available to other major powers, including the borg
But what good is vastness? Kirk's midget ship easily defeated the giant. Granted, it was supposed to be a ruse. But perhaps it was a double ruse, to hide the fact that the ship had no other superpower but the ability to make all that bulk move at moderate warp.
An ancient alien civilisation with the ability to warp reality with the power of their mind, which despite being commonplace in the trek universe makes them a singular threat that requires their planet to be quarantined under threat of death. That the last contact with this species ended on friendly terms shouldn't in any way suggest they might be a potential and powerful ally.
In what sense, though? They hold the remote to a good TV set, but they don't really know how to fix that set, or to build another. Perhaps rummaging through their planet helps the UFP make better holodecks (which they eventually have, surpassing the Talosian illusions in realism), perhaps not.
But storing the Talosians in a vast warehouse to be studied by Top. Men. isn't the issue in this particular case. The issue is why Pike was the first to fly close to the Talos system. Supposedly, it's a good shortcut from Rigel to Vega, but perhaps no good for anything else (c.f. the long-abandoned wasteland in which Khan was found), so humans just plain won't get there until shortly before DSC.
Then again, dozens could have gone to Talos before Pike did. The
Columbia supposedly really did, and didn't survive to tell the tale. This could have happened innumerable times; all that nonsense about the Talosians "not realizing humans weren't good slaves because they had not met any before" is an obvious lie, an attempt to save those frail necks when Pike (that is, Number One) has them on a stranglehold for once. Ditto for Vina being "disfigured" because of "no knowledge of humans"!
Timo Saloniemi
. While letting them retain the French language only to be used occasionally for effect.
