Default it to on.

Apple did that with their tracking preventer and something like 96% of people left it that way.

Off would be rare, and those people would be the ones who wanted to be tracked anyways.

Apple's App Tracking Transparency (if that's what you're referring to, as opposed to Intelligent Tracking Prevention) doesn't even default to anything. It asks you and gives you two equally-prominent options, but indeed even in that case the acceptance rate is still just 4% which I assume is either misclicks or ad-tech people.

I use TextNow sometimes and it asks me to allow tracking so it can remain free. It asked nicely so I allowed tracking.

MS defaulting it to on is pretty much what killed the whole thing and everyone started ignoring DNT.

Sure, it only works with corresponding regulation imposing consequences for ignoring it.