GTA 6 Trailer Visuals Won't Match Final Game, Ex-Dev Warns
While this is a disappointing thing to hear about Grand Theft Auto VI before its massive launch later this year, it does make sense. There have been extreme examples in the past, such as the original Killzone 2 trailer for PlayStation 3 at E3 being entirely pre-rendered footage that couldn't even run on the console. But it wouldn't appear that such a drastic change is ahead of us for Grand Theft Auto VI—Rockstar has a good track record with pre-release footage so far, and the footage is certainly in-engine.
What it does communicate instead is like the controversy from Spider-Man's PS4 release. Back then, Redditor u/Ghulam_Jewel posted the below image to Reddit, pointing out shrunk puddles and a seeming loss in graphical fidelity between the E3 trailer and the final game. An Insomniac developer later claimed that the difference was down to a puddle being shrunk and day lighting conditions being changed for that particular mission, but gamers assumed the differences were down to graphical downgrades for the sake of performance (i.e., reducing the number of reflections in the scene.)

David O'Reilly's words to Kiwi Talkz ahead of Grand Theft Auto VI seem to refer more to minor adjustments than, say, nixing features like ray tracing from the final game. But until we actually see it running on real hardware for ourselves, who knows? Maybe Rockstar Games did get a little too ambitious for its own good with early trailer footage, and launch or yet-to-be-seen Trailer 3 footage will be a noticeable downgrade.
For the most part, we remain optimistic that the final version of Grand Theft Auto VI will look and feel like the AAA game it is. In the worst case, the eventual PC version should be enough to fill in whatever gap there ends up being between trailer footage and console footage, if one even exists.
We will have to see, but Rockstar has a good reputation for setting realistic expectations for its games ahead of release, and delivering on those expectations. Even the launch version of Grand Theft Auto V looked pretty good, and that had to run on the original Xbox 360.
Image Credit: Rockstar Games, Insomniac Games, u/Ghulam_Jewel on Reddit