If I recall correctly, FH3 had its post-launch additions leaked after a developer build of the game was accidentally pushed to a stable branch. Similarly, I believe FH4's vehicles weren't hidden very well during the preload for that game, so dataminers made quick work of it. My mind is quite fuzzy though, this was years ago so I could be misremembering. I recall most of FH5's leaks being post-launch and mostly comprised of images found in the game files. Again, could be misremembering.
There's still plenty of time for them to reveal cars though so I wouldn't worry too much on that front. What I really wanna know about more right now is if FM8 vinyl groups are going to be able to be ported. My main worry is that they won't allow it since you could straight up steal other peoples' vinyl groups in that game for a little while. So the natural worry here is "well maybe they don't want to risk stolen vinyl groups being ported over".
Some further clarification on why we can't port entire liveries would also be appreciated. Pretty sure it's because they redid the UV's for each vehicle but it would be nice to have a confirmation.