Looks like a helpful honeypot! Any tool that will public announce support for this spec is a tool I know to avoid because it collects telemetry without explicit opt-in in the first place.
Example: the software crashes, and there is a crash handler that asks you if you want to send a crash dump. With DO_NOT_TRACK, the crash handler is disabled entirely, no question, no dump.
If it gets some adoption, that's probably how it will work. Those who have an financial interest in using tracking (ex: ads) probably won't support such an option.