This extension does not collect, transmit, sell or share data and makes no network requests. It stores only two on/off preferences locally in your browser. It inspects supported Google page headers in memory to hide promotional controls; it does not record or transmit page content. Calendar access is optional.

Full privacy policy: https://github.com/maximtop/hide-gmail-upgrade-button/blob/master/PRIVACY.md
Contact: me@maximtop.dev