DNT was originally proposed by the ad networks and they promised to honour its setting, secretly hoping that nobody would enable it. Browser makers then promptly implemented it, defaulted to on. Ad networks whined "not fair" and then went back on their promises and refused to honour it. It has therefore mostly been ignored (though my own services pay attention to it).
Most annoyingly, Safari dropped support for DNT altogether, after being one of the first to adopt it. This was a short-lived victory for the ad networks; Apple has just got their own back by implementing their privacy first approach to iOS, and it takes control away from ad networks completely.
One of the fundamental problems is that you can't tell if a site honours it or not. As a result, while the Tk header is well intentioned, it's entirely dependent on the honesty of the provider, so it has about as much hope of ever being implemented faithfully as RFC3514.