The X-Client-Data header is for evaluating the effect of Chrome changes, not ad targeting: "Additionally, a subset of low entropy variations are included in ne

The X-Client-Data header is for evaluating the effect of Chrome changes, not ad targeting: "Additionally, a subset of low entropy variations are included in network requests sent to Google. The combined state of these variations is non-identifying, since it is based on a 13-bit low entropy value (see above). These are transmitted using the "X-Client-Data" HTTP header, which contains a list of active variations. On Android, this header may include a limited set of external server-side experiments, which may affect the Chrome installation. This header is used to evaluate the effect on Google servers - for example, a networking change may affect YouTube video load speed or an Omnibox ranking update may result in more helpful Google Search results." -- https://www.google.com/chrome/privacy/whitepaper.html https://www.google.com/chrome/privacy/whitepaper.html