DNT is a flawed idea in the first place. DNT is based on the idea that users can opt out of being tracked by sending that header. But (e.g.) European privacy law requires having users explicitly opt in to being tracked -- the message DNT is meant to assert is the legal default under GDPR.
DNT only works if the assumption is that users who make no choice can be treated as consenting (which is no longer the case under GDPR). But if you set DNT by default you're not asserting "this user doesn't want to be tracked", you're just making it impossible to tell whether the user explicitly opts out or hasn't made a choice (and therefore actually do consent).
If tracking is opt-out rather than opt-in (i.e. if we disregard GDPR and similar privacy laws and go with how US startups have operated so far) that means DNT is no longer a reliable signal for opting out and thus meaningless.
Implications about consent and privacy aside, DNT only works if it is used with intent. In the absence of intent, by making it the default without the user's knowledge, it becomes ambiguous and therefore pointless.
To put it differently: if there had never been any browsers that set DNT by default (except maybe browsers explicitly marketing themselves as "privacy first" like Brave does), you could use DNT as an explicit assertion that you do not consent to being tracked. This means it could actually serve as a technical implementation to opt-out of any "implied consent" allowed by the GDPR and making use of your right to control your data.
But thanks to Microsoft randomly slapping on the header to piss off Google, DNT is now too ambiguous to infer any of that.