If you bought the game before it was delisted, you can still download, reinstall, and play it (both online and offline) as usual.
While the game was still active, there were occasionally free car packs, such as the Mitsubishi Car Pack, though these required owning the base game. Risks of "Free Download" Sites Forza Horizon 4 Free Download (Steam Edition)
There are only a few very specific ways users might have obtained the game without a direct purchase: If you bought the game before it was
There has never been a permanent "free to keep" version of the full game on Steam. Any sites offering a "free download" for the Steam Edition are typically providing unauthorized pirated versions or scams. Rare "Free" Legitimate Methods (Past & Current) you can still download