In January 2019 W3C Tracking Protection Working Group concluded work on Do Not Track standard citing "insufficient deployment of these extensions" and lack of "indications of planned support among user agents, third parties, and the ecosystem at large." In February 2019 Apple Safari 12.1 was released without support for DNT to avoid it being used as a "tracking variable."
You can tell how bad the lack of support / teeth is when people start using the flag to not track them as an extra way of tracking people. That's extremely telling... but sadly not unexpected by many of us.
Should Mozilla remove the DNT header from Firefox like Apple did in Safari? When DNT does nothing except give trackers one more bit of fingerprint entropy, is there any value in users still allowing users to send DNT? "DNT: I am one of those people who does not want you to track even though I know you will."
There are millions of Do not Trespass signs out there, the difference is that they have some legal weight.
Ten years ago it would have been a different matter, but it doesn't seem that far fetched to get do not track to be the legal equivalent of a "no" on those GDPR consent forms, but with no options for dark patterns and no way to re-query on every page load for those who opt-out.
I bet the ad industry wishes they'd played ball, now that browsers are baking tracking protection in.