if itch.io stops hosting it you would have to be part of the discord server you get when you purchase it but i am sure you could email the dev with a copy of your receipt and he will probably give you the discord to have access in the event you didn't join

Oh yea, its unlikely that the game will go away, but if that ever happened, then yes, Dev wouldn't be able to update the game because he would lose access, and you wouldn't be able to download it because it would be banned.
Although even then, the dev still have alternatives to sell the game such as Fanbox and SubscribeStar, those who lost access to the game can contact the dev, he will verify your itch.io purchase(if you made one) and will give you a redeem code from the other sites, if you really want to be extra careful Then join the discord group found in a zip file you can download after you purchase the game(its called RoomLinks.zip).

With all that said, Put your mind at rest, the game is not in any risk of getting banned.