Hmm, I remember there being an option to allow Steam keys even though it's free. Try going here and see if this will repopulate: https://itch.io/docs/buying/already-bought
Meanwhile, I'll look into what happened.
Viewing post in Assassin at Crimson Keep comments
I'm inclined to believe there's no Steam key option available for those who obtained the game during the free promotion, likely by design. I would appreciate an update to the main game page to let people know that the Steam key is only available to those who paid for the game. Thanks so much for the temporary free giveaway and I look forward to my playthrough :)