itch.io

I'm sorry, but no. I can't give Steam keys to all people who buy the game on different platforms, otherwise most keys will just end up getting resold. I am not saying that I think you intend to do it, but it's the reason behind the principle.

However, Itch.io version will receive all the same updates as the Steam one, so you really aren't missing out if you own one and not the other.