Data
iOS 14.5+
Apple's App Tracking Transparency framework, launched April 2021, requiring apps to get explicit user permission before tracking activity across other apps and websites. The majority of users opt out. Devastated Meta's tracking accuracy, substantially reduced retargeting pool sizes on iOS, and degraded conversion reporting reliability. Forced the entire industry toward server-side tracking, probabilistic modeling, and first-party data strategies. The single largest disruption to digital advertising measurement in the past decade.
Why it matters
It is the origin of most modern attribution loss. Opt-in rates well below half mean platform-reported conversions became a sample rather than a census.
In practice
Treat the pre and post periods as incomparable. A ROAS decline dated to 2021 is usually measurement, not performance.
Related terms