From Ars Technica:

96% of US users opt out of app tracking in iOS 14.5, analytics find

When consumers are given the explicit choice as to whether to allow companies to track your activity across the web, not very many people are OK with it. Although the 4% figure is on the low end of the optin estimates, acceptance and perhaps awareness of such tracking as a norm was low.

Facebook is of course one of the more famous exploiters of this kind of technique. Supposedly Apple’s inclusion of a prompt that made their tracking opt-outable when done on an iOS device cost them $10 billion.