We could just as well decide the heroes simply stopped doing it; nothing indicates they would be unable to do it. Picard and Riker in "Time Squared" discuss time travel as a known quantity:
Picard: "What force or phenomenon could cause the shuttle to be thrown back in time?"
Riker: "None that we've encountered. In theory, accelerating beyond warp 10."
Picard: "Using the gravitational pull of a star to slingshot back in time. Is that what happened here?"
It sounds as if
a) the TNG heroes have encountered several all-new time travel techniques themselves
b) none fit the specs of this particular incident
c) the underlying concept of time travel in Newtonian space is that you travel faster than infinitely fast - so you arrive before you left - and Riker refers to this, half in jest
d) slingshotting is a different way to time travel, and one Picard seriously considers, but doesn't fully accept as the answer to the mystery of the day.
Overall, though, time travel in TNG, DS9 or VOY is mundane and treated matter-of-factly.
This is in sharp contrast with, say, natural phenomena that travel at warp (often seen in TOS and retroactively in ENT, but emphatically declared an unknown quantity or an impossibility in TNG) or dark rifts in space (something Kirk met in "Immunity Syndrome" but Data in "Where Silence Has Lease" describes as previously unencountered)...