Creators on itch.io are discouraged from deleting their games once they have at least one buyer. They'd have to contact support and give a good reason. However, it's possible to get around that by deleting every file, and it's hard to stop them because creators also need control over their uploads for legitimate reasons. It's best to keep your own backups for that reason.
If any creators are reading this: if you want to take a game off the shelf, you can do that more easily by ticking the "Disable new downloads & purchases" box under the public access setting. That leaves the game available for those who already bought it.