Subtle tab does not track any user-specific data. It uses Google Analytics to save aggregated events to improve user experience based on what features are used more. If still, you want to opt-out of Google Analytics tracking, please visit http://tools.google.com/dlpage/gaoptout or you can set up a filter in Adblock Plus or similar ad blocker tools like AdBlock, uBlock or Adblock Pro.

For detail: https://www.subtletab.com/#/privacy