Perhaps they were available without the DLC? I can't really comment on it as this is the first time I've heard of there being a problem.

I don't quite understand what you mean. Afaik there's never been an issue with reposting models from the game as long as they can't be loaded without the DLC they came from. The workshop is full of tons of edited vanilla buildings, which are copyrighted by the game and for years both me and SvenBerlin has had some of the default vehicles on the workshop as props. There's even models from Cities in Motion, which CO/Paradox are aware of.