Employee here- I haven’t seen an official comment here yet
The JSON file that was linked to does have partner domains which (when the header is present) the website will provide a specific integration. When those specific partner sites are visited, the headers are sent with the request
An example someone mentioned here already is marketwatch. They have a promotion where you can sign up for a free subscription if you use Brave
The browser is open source and nothing is being hidden- although this and the whitelist (used for a better webcompat experience) could be better documented
Should these lists be shown in the UI and configurable? (ex: disableable?) I wonder what a better experience would look like for people that don’t want this functionality