What would keep stuff like GMT donor and backer furniture from being obtainable through workshop? Even if the public stuff gets deleted, people could probably still upload the model privately for their own use.
One of the things that will prevent this is having a option when right clicking on someone's furniture, which will tell you if the item is genuine or not. If it's not genuine, it's from Workshop and the player didn't obtain it from the game.
On top of the right click thing, the tool tip itself will say "Workshop Item".