I'm not familiar with what part of the site you're talking about. Is it possible to show a screenshot?
If you're talking about the search input in the red header on the top of every itch.io page, we don't have autocomplete there. So if you're seeing autocomplete it's coming from your browser.
Edit: We now have autocomplete in the search bar, more information can be found here: https://itch.io/t/3361091/search-autocomplete