The existing Save-Data header will be made a formal client hint by adding a new CH-Save-Data permissions policy that controls its delegation. This hint will still be sent by default (when lite mode is on) and delegated to all first and third parties by default.
The current `Save-Data` header is sent when a browser or operating system data saver setting is on (e.g., Lite mode) for all first and third party requests and lives outside the client hints system. `Save-Data` will be made a proper client hint and its delegation to third parties could be prevented via a new permissions policy `CH-Save-Data'. It will only be sent when data saver settings are on, unlike other client hints.