Nothing is stopping existing owners from downloading the offline installer backups, but existing owners will still have access to their games through GOG regardless. Delisting just means that the games will no longer be purchasable, so if there's a game you want to "own", as long as you buy it before it's delisted, it will remain accessible to you for the foreseeable future.

literally nothing. Most of the "repacks" I have seen are made from the gog release.