Devs are generally discouraged from taking their work off the site, but at times it can happen. Itch.io is giving each dev a lot of control, which is what makes the platform so easy to use, but that might come with some downsides.
A good advice in general is, when you purchase a game, you can download a local copy of it, which you can keep forever.
Donating extra money is always an amazing gesture, but comes without any guarantees, as the dev does not have to keep working on a game.
I’m not sure what can be done in cases like these, but contacting support would be the best bet. They’ll be able to give you more details and next steps, if any.