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…
The only way to stop tracking is via laws or regulations. Technical solutions are, arguably, a never-ending arms race - probably a losing one for end users.
DNT was a way to demonstrate consumer interest in not being tracked, and it put businesses in the position of ignoring explicit requests from consumers for privacy.
Unfortunately, nobody effectively capitalized on that.