I got the game on Itch.io in 2016 (or later i forgot) and now i just figured out how to get the steam key, but now its having problems and i cant get the steam key even know i already got it.
The steam key will not expire. There are steam keys in the itch.io database, so that there shouldn't be any problems.
Maybe this blog post will help you: https://itch.io/t/67604/how-to-get-a-steam-key-from-your-itchio-purchase
If you are still having problems, please contact the itch.io support.