The initial results of iOS 14.5 opt-in ad tracking are in:
4% of US users actively opting in (96% not)
12% of international aka non-US users opting in (88% not)
Per Daily iOS 14.5 Opt-in Rate [flurry.com] (as of 07-May-2021, 2-weeks in)

To those of us who remember the 2009 study that found 8% responsible for 85% of clicks this is not particularly surprising.
See Updated “Natural Born Clickers” Study [comscore.com]

Very few SEO/webdev at the time of the initial 2007 study release nor the more widely commented 2009 update ever bothered to research the demographics of ‘natural born clickers’.

After all, what value could there be in identifying, grabbing, and retaining those few responsible for so much? Yes, that was rhetorical snark.

Odds on a more proactive behaviour this time around?

Note: the popcorn level best part of this Apple ad tracking opt-in rather than opt-out-if-you-can are the FaceBook hysterical claims that if iOS users don’t opt-in FB won’t be able to stay free to use...