I was at Mozilla when this was implemented. It was completely optional for websites to support this. A few did at first. A lot of people internally wanted it to be on-by-default, but the argument was that if it was, nobody would respect it – after all, what tracking platform would willingly only track the 0.1% of people who went into the settings to enable it? (Internet Explorer did eventually enable it by default, w…
I worked at one of the big adtech companies at the time. DNT was a carefully negotiated compromise between the ad industry (and by proxy, the sites that depended on it for their income), the browser vendors, and privacy advocates. We implemented DNT it in our edge infrastructure and were ready to deploy it. But then Microsoft broke the agreement by enabling it by default, as part of their war with Google (and after t…
What also would be legal is to offer the user a choice at the first startup.