iOS 10 Zeroes the IDFA for Opted-Out Users

What Changed

Since iOS 6, Limit Ad Tracking had only been a flag: apps could still read the IDFA, and Apple's guidance let them keep using it for limited purposes, including frequency capping and attribution.

With iOS 10 in September 2016, Apple changed that. For users with Limit Ad Tracking turned on, the IDFA now returned a string of zeros. There was no identifier to use at all, for any purpose.¹

Apple didn't discuss the change at WWDC that June, as Eric Seufert of Mobile Dev Memo noted when he wrote about it in August.²

Why It Mattered

It turned an opt-out from a request into a technical fact. Before iOS 10, a company could honour the flag or ignore it, and many used the ID for attribution regardless. After iOS 10, opted-out users simply couldn't be tracked with the IDFA.

That affected more than ad targeting. Without an ID, ad networks couldn't cap how often an opted-out user saw the same ad, and attribution providers couldn't connect an ad to an install for those users. Analysts at the time, including Eric Seufert at Mobile Dev Memo, pointed out that the change hit measurement as much as targeting.²

At the time, only a minority of users had turned on Limit Ad Tracking, so the overall effect was modest: about 20% of US iOS users by October 2016, up from 16–18% before iOS 10, according to the attribution company Adjust, as the Future of Privacy Forum reported; another measurement company, TUNE, put it at 17%.³ But iOS 10 established the mechanism Apple would use in 2021: when a user says no, the IDFA becomes zeros. App Tracking Transparency made that the outcome for everyone who declines. Google took the same step on Android only in 2021, when it began zeroing the Advertising ID for users who delete it.