Should (imho of course) try out the game unmodded at first anyway, at which point you'd learn what the default game has to offer, then (if you want to) download mods to enhance your gaming experience.
This way you'd know which parts come with the default game from top of your head.
The reason I don't really like this idea as is, is because it would add nothing to the game except to break immersion when using mods.
Imo a good mod enhances a default game seamlessly.
I don't really see how this would actually benefit anyone except for people new to the game who rush and install a bunch of mods and then (for whatever reason) would want to know which parts are from the default game. The use of such a feature for basically anybody else would be next to meaningless. And it would only add dev time which could be spend better someplace else.
Or at least that's my first thought of this suggestion. It doesn't really benefit anyone except for the few who don't really bother to want to learn the game anyway, so why cater to them?