App Tracking Transparency and the IDFA

App Tracking Transparency is Apple's prompt asking users to allow tracking across apps, and the IDFA is the iPhone's advertising identifier that now only works if the user says yes to that prompt.

You remember when the IDFA was the backbone of iOS measurement, a stable device ID you could pass to any network for clean, deterministic, user-level attribution. Then Apple shipped App Tracking Transparency, and now every app has to show that "Ask App Not to Track" prompt before it can read the IDFA. Most people tap decline. When they do, your app gets zeros instead of an identifier, and the cross-app tracking you used to rely on simply does not happen for that user.

The practical result is that deterministic, ID-based tracking across apps on iOS largely collapsed. You can still get the IDFA from the minority who opt in, but you cannot build a measurement strategy on a signal that most of your audience refuses to hand over.

Stay honest about what actually broke. The IDFA is mostly gone in practice on iOS, but tracking is not impossible and privacy is not dead. The industry's response was to lean on SKAdNetwork and on first-party data you collect with consent, not to hunt for some clever workaround around the prompt. So you treat opted-in IDFA as a bonus, not a foundation, and build your iOS reporting on SKAN plus your own logged-in events.

Assume the IDFA is gone for most of your iOS users and design your measurement as if it never comes back.