While not exactly be sure, with InfiniteQuestion's situation, I believe he released livery support for every single vanilla car. I agree that moderation team can not magically know all of those cars were modded. But expecting a question from modders to ask "I'm thinking about making a mod like this, do you think it would be allowed?" is not actually correct either. Because by the nature of the mod, following up with the enhusiasm of doing a big project and hoping people will like it very much, it is impossible to think of something like this at the very beginning
Not only is it entirely reasonable to ask that ppl think before doing, it's also -- especially considering the amount of work put in -- simply the common-sense thing to do.
People tend to shun staff, overall. That's a normal human notion. Like they want to stay away from the Police. And that's really where the analogy ends, as we're NOT the Police: we just exist to make sure 5mods runs smoothly, and doesn't get into trouble with the IP-rights holders of the game and/or other modders.
So, I reiterate, if anyone wants to embark on a Project of such megalithic proportions, with an 'All vanilla cars', or 'All vanilla peds' mod, please come to to us first, and we can assess the situation, prior to a possible major frustration. And the latter is exactly what happened to InfiniteQuestion: he first releases all vanilla cars, and only then, when we forbade that, he was so frustrated that he tried a way to get his mega-pack uploaded regardless, but meaning he had to do considerably more work to comply with our ruling regarding visibility -- an offer which he ultimately didn't take.
Tl;dr: come to us first. We're here 24/7 (not all of us are around 24/7, of course, but there's always someone around).