Shelf Zero's privacy policy: https://shelfzero.app/privacy

In short: the extension stores only your Shelf Zero access token (in browser storage) and, when you explicitly save a page, sends that page's URL and title to your own Shelf Zero account at app.shelfzero.app. It reads the active tab only at the moment you use it, contacts no other hosts, keeps no browsing history and shares data with no third parties.