Maybe the whole slingshot thing is classified a century later? Obviously Kirk and company know how to do it. If it was common knowledge, everybody would be forever altering the timeline. There are other Voyager episodes where ye olde slingshot would have solved a problem or two. There’s also the whole “don’t alter history” thing, which is why Sisko gets a visit from Temporal Investigations after his little side trip into the past.

I can’t recall how the Enterprise E even got back home in First Contact, if they addressed the issue at all.
JEDIT:Geordi says he can recreate the Temporal Vortex thingy, which seems a better way to get around than ye olde slingshot, but it’s probably never mentioned in Trek again.