I remember a TNG episode where some scientists said that warp drives were causing a lot of damage to subspace. At the end of the episode Star Fleet decided that warp speeds would be restricted to warp 5 except for emergency situations.
Of course the next episode they were cruising along at warp 7.
Is this ever mentioned again in Star Trek?
It was never addressed in canon.
The episode to which you refer, "Force of Nature", occurred in the last season of The Next Generation at a time when most of the effort in storylines was geared towards Star Trek: Voyager and Deep Space Nine: the developers of those shows didn't particularly focus too much on the continuity issues introduced as The Next Generation was wrapping up. Indeed, even The Next Generation writers ignored or found ways around it for the remainder of the season.
However, according to Memory Alpha:
According to the unpublished VOY Season 1 edition of the Star Trek: Voyager Technical Guide, by Rick Sternbach and Michael Okuda, it was suggested that because of the variable geometry pylons, warp fields may no longer have a negative impact on habitable worlds as established in this episode.
But since the manual was never published, it's not canon. Alternative theories include:
It's also interesting to note that the issues raised in "Force of Nature" are referenced, indirectly, in the last season Voyager episode, "Renaissance Man", when the Hierarchy attempt to trick the Voyager crew into ejecting their warp core because they entered an area that's occupied by people who believe warp drives disrupt subspace.
Early in the episode The Pegasus Picard is given orders to search for the titular ship. The admiral who gives him the order also gives him authorization to ignore the warp speed restriction, for the course of the mission. This episode was 3 episodes after Force of Nature, the episode where the 'speed limit' is introduced.
It was never mentioned during DS9. During VOY, they may have mentioned it during a couple of episodes where there are regions of space that are unsafe to traverse via Warp Drive (here and here), but I don't recall offhand.
The short answer is that it's not addressed again, beyond the end of TNG.
Quoting from the Star Trek Encyclopedia (2nd Edition)
The council further ordered Federation ships throughout the galaxy to restrict their maximum speed to warp 4.7. (TNG: Force of Nature). The Starship Enterprise-D and other ships observed the "warp speed limit" for the remainder of Star Trek: The Next Generation. After that, it was assumed that Starfleet was able to develop a more "environmentally friendly" warp drive, so the speed limit was evidently lifted.
One can assume that if the fix was actually quite trivial, the other major powers would have had no qualms about enacting it.
As to why the decision was made to simply gloss over the problem, the simplest way to look at it is that continuing to insist upon 'warp-speed restrictions' would have demanded regular explanation (adding to the length of scripts) without actually improving them in any way.
As Ronald D. Moore stated in the Captains' Logs: The Unauthorized Complete Trek Voyages book:
...when you limit warp drive, the rug is being pulled out from under Star Trek.
They very clearly stated that warp speed was restricted in areas of space affected- it was never restricted across all areas of space. It is assumed that any time they say "authorized to exceed warp speed restrictions" it's because they're in an area of space affected by the rifts.
