Auto-saving is still uncommon on web, compared to mobile apps. The default architecture is not built in a way that it's expected behavior. Users want to know its saved. Instead of making a fake button that actively misleads users, there should just be some small visual notification showing saving/saved. You don't know how long a save takes. People just need a visual indicator that it's safe to close the tab. It's easier to just tell them the actual status of their save.