So, what happens to the donators? They have to pay to have access again?
I can't speak for the developers but I'm sure they'll figure something out if they're fair (I would). Like if you donated some amount of money for the game in the first place (some percentage of what they might finally charge for it on Steam) you would get a free Steam key to get it from there. The problem with Steam is you can't charge whatever the user wants like here so it's a fixed amount, and the developer hasn't said anything about what that would be or how they'll handle itch.io users. I'm sure that's part of all the logistics they'll work out when they move to Steam, it's just a matter of time.