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."