No, a full-on Le Mans racer was not street legal. Like
@VXR said, it's "Ferrari being Ferrari." For some reason, they don't like their cars in video games to have non-Ferrari liveries applied to them. In a special edition of Forza 6, you could get 10 cars with Xbox liveries celebrating 10 years of Xbox. The 458 was one of them, but unlike the others, it was simply painted black.
There's also a thing in Forza where sone Ferraris don't have access to the full collection of manufacturer decals, just the Xbox-related ones.
Lastly, despite what I and others have said, Forza includes 3 versions of the 458 GT3/GTE/GTC, and they all have either AF Corse of Rizi Competizione race liveries.
Sorry for the heavy use of Forza info as evidence. That's where most of my knowledge is.