A ‘Clear-Site-Data’ HTTP header prompts the user agent to clear browsing data associated with the requesting website. The supported browsing data types are cookies, storage (i.e. “site data”), and cache. This is a privacy and security enhancing feature. A sensitive website can trigger local data deletion after the user signs out. A website dealing with a persistent XSS attack can use this to ‘reset’ itself to a clean state.
Docs: https://docs.google.com/document/d/1I6m4QwbTNhG6wdtazamhTnArJN-UMUGqpvwH6InBEaM/
Samples: https://github.com/w3c/webappsec-clear-site-data/tree/master/demo