There is no official method from Apple to track app uninstalls — this would have user privacy implications, and Apple has built their reputation around protecting user privacy. Given the history of public outrage around this sort of tracking, it's unlikely Apple will ever allow it.
AppsFlyer (and other companies that claim to provide 'uninstall tracking') use silent push notifications to do it. Essentially, they ping the device regularly and report an 'uninstall' if the push notification is not successfully delivered.
There are a couple of major issues with this approach:
- It's unreliable. If the user doesn't allow push notifications, or turns them off at some point after installation, you'll get drowned in false positives.
- It's against Apple's rules. The App Store Review Guidelines say this:
4.5.4 Push Notifications must not be required for the app to function, and should not be used for advertising, promotions, or direct marketing purposes or to send sensitive personal or confidential information. Abuse of these services may result in revocation of your privileges.
AppsFlyer actually got in some public hot water for this product last year, and I've heard through the grapevine that Apple intends to crack down on similar services at some point in the near future.
Ultimately, an uninstall is little different from a user who has the app installed but never engages with it. You're probably better off focusing your efforts on building an awesome user experience that improves acquisition and retention, rather than worrying about how to measure a metric that is specifically on Apple's blacklist.