[1] You can read the text: https://eur-lex.europa.eu/eli/reg/2016/679/oj there is a single instance of "cookie" (in the preamble) and no instance of "banner".
While DNT could potentially be used for opt-out, it wouldn't comply for opt-in because it is not specific or informed as the user does not know what specific data processing activities will be done, can't opt-in or out of specific data processing activities, doesn't know the identity of those doing them or that they can withdraw consent at any time.
There are proposed browser signal specifications that would meet legal GDPR consent defintions. See https://www.dataprotectioncontrol.org/spec/
And as for why DNT did not took off, it's because MSFT sabotaged it by making DNT set by default in Internet Explorer. The social contract in that time between adtech, publishers and users was that the signal would strictly be opt-in. The adtech industry used IE making DNT the default as justification for not honoring any of the signals being sent by browsers. It doesn't take a lot of reasoning to realize MSFT did this on purpose, knowing it itself earns income from ads.