Would robot traffic be considered tracking in light of GDPR standards? As far as I know there are no regulatory rules in relation to enforcing robots behaviors outside of robots.txt, which is more of an honor system.
DNT and GDPR was just an example. In a court case about tracking, DNT could be found to be a clear and explicit opt out. Similarly, in a case about excessive scraping or the use of scraped information, robots txt could be used as a clear and explicit signal that the site operator does not want their pages harvested. It all but certainly gets rid of the "they put it on the public web so we assumed we can scrape it, we can'task everyone for permission" argument. They can't claim it was "in good faith" if there's a widely-accepted standard for opting out.