I thought it still existed in the 31st century in Enterprise? And I think the point of the "cover-up" was to say "ignore the last two years, if you want".
With that kind of reasoning, any kind of technological advancement would be kept under wraps. Cro Magnon Chief: "Hey, we'll forget about fire, because someday a nearby tribe could use it to attack us!!"
Indeed it did. Now of course Discovery could have ended up outside Federation territory, and may not even go near it during the course of the 3rd season, but it seems clear that the Federation has not, in fact, fallen. Things like the Relativity and Daniels' group are all the proof we need.
I thought slingshot time travel was discovered (or refined to be useful) by the Enterprise in Tomorrow Is Yesterday?
The least practical way of traveling through time is the Gate (aka giant donut) Of The Guardian Of Forever.
I seem to remember (I haven't watched that episode for some time) that Spock said that the guardian was neither truly alive nor exactly a machine or something to that effect.
Of course the Q might have built the Guardian, as implied in one novel The rascals. No wonder the Guardian speaks in riddles.
Here's an unpopular Trek opinion: I'm rather apprehensive about the new Lower Decks series. I don't think I like the idea of turning Star Trek into a joke. Even the original TAS still played it straight...I can't say I'm looking forward to the wacky, zany Lower Decks. May have to give this one a pass. Still thinking about it.