I know there are free games such as Team Fortress 2 that have trading cards. Steam games that are free usually don't have trading cards so people don't just add them to their library to farm trading cards and get free money. TF2 only allows trading cards to drop when you purchase something from the ingame store.
I want to know if there was ever a free game on Steam, where you can add it to your library and get trading cards without paying anything ingame. The way I imagine this happening is if a game was free for a limited time (maybe on release or later) and when you add it to your library, you get the trading cards as if you bought the game for real. Has there ever been such a case? I'm just curious to know how Steam handles trading cards in temporarily free games.
Edit: Not a duplicate to this question, as it deals with free demos, not full games that happen to be free. Free weekend games, where the game goes back to being paid-only after the weekend, also don't count, as those are not added to your library when the weekend is over.