I am not talking about all the whole DLC, I mean any part of that content.
What you mod for you own use is up to you, though at this moment I doubt that the DLC (1990's) will be moddable, but I could be wrong.
Anyway, let's say that you have managed to create a 1990's car or track mod, using the files from the Classic DLC. If that mod can be uploaded, installed and used by anyone who ONLY has the Standard Edition game, then that mod would not be allowed. It doesn't matter whether if you happen to own the DLC, it matters if the original files from that DLC
can be used by those that don't own the DLC.
Example: The Classic edition contains these 2 tracks, Estoril and Imola. If anyone modifies those 2 tracks so that they can be installed and used in the Standard Edition, then that's a no-no.
The same would apply if you took the 1999 Ferrari F399 from the Classic Edition and modified it in such a way that it can be used by someone with the Standard Edition only.
Naturally, if a mod contains Classic content, and can
only be used by someone who has the Classic Edition, or has upgraded to Classic via DLC (because CM have technically restricted the game in some way) , then there is no problem as far as I can see.
Until we know details about the Classic content and how it
can or cannot be used (technically), really we are just blowing hot air here until release. It might very well end up as @
Bram Hengeveld implied above, no classic mods at all, just standard 2013 mods