In Heat is a fan made game of FNAF, and they planning on adding it to Steam when they done with the game.
The only thing that can prevent it is if the developer of the original FNAF thinks its violating copyrights.
Other then that, i dont see any reason for them not to add it on steam. But likely only when its done first.
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Because of Patreon members are funding enough on the game, Itch gets to have it for free untill its complete by logic, maybe. My guess is that the price will be between 5-20$ on full release, both on itch and likely also on Steam, if it becomes possible for them to release it to Steam.
Unless they decide they dont want any money of the game when its complete, its possible it will stay free even after its complete. Since i dont know anything about how steam handle games, it can be that they have to price the game on steam so its validated on the store.