ConcernedApe has never released a free full version. However, Steam Remote Play Together allows a friend with a legal copy to let you play for free (streaming). This bypasses the need for a compatibility version entirely.
Steam does not have a friendly dropdown menu, but you can download older compatibility versions for free using the Steam Console. stardew valley compatibility version download free
💡 The only legal way to play Stardew Valley is to purchase it. However, the compatibility "beta" branch itself is a free utility provided by the developer for everyone who owns the game. ConcernedApe has never released a free full version