There are also reports that Apple has tightened restrictions on 'tracking by app' from iOS 14.5 or has already refused to update

Apple is reportedly refusing to update apps that don't comply with the iOS 'App Tracking Transparency' rule. This move is believed to be due to the stricter privacy rules for iOS 14.5, released in the spring of 2021, than previous versions.

App Store now rejecting apps using third-party SDKs that collect user data without consent --9to5Mac
https://9to5mac.com/2021/04/01/app-store-now-rejecting-apps-using-third-party-sdks-that-collect-user-data-without-consent/

Apple Now Rejecting App Updates That Defy iOS 14.5 App Tracking Transparency Rules --MacRumors
https://www.macrumors.com/2021/04/01/apple-rejecting-app-updates-tracking-transparency/

Apple rejecting apps that collect data for'device fingerprinting' | AppleInsider
https://appleinsider.com/articles/21/04/02/apple-reportedly-rejecting-apps-built-with-sdks-that-incorporate-device-fingerprinting

App developers can identify users on the Internet by 'device fingerprints ' generated from information such as device OS, version, time zone, IP address, etc. in the app, and perform targeted advertising and analysis. Is possible.

Apple doesn't ban device fingerprints themselves, but starting with iOS 14.4, released in January 2021, the app can't track ads without user permission.

Apple tests pop-ups to limit ad tracking in beta iOS 14.4-GIGAZINE

If you use the iOS SDK , which is a software development kit (SDK) for apps for iOS, there is no problem because it complies with Apple's privacy guidelines. However, some third-party SDKs do not comply with Apple's privacy guidelines, allowing apps to get device fingerprints and track users by bypassing user permission.

Meanwhile, it is reported that Apple has begun refusing to update apps that do not comply with the 'App Tracking Transparency' rule. Aude Boscher, marketing product manager for French ride-hailing service Heetch, said on the industry's Slack channel, 'Our app has been rejected by Apple's app reviewers due to the MMP SDK that creates device fingerprints. He confessed.

Several other apps besides Heetch refused to update with the message 'This app uses algorithmically converted devices and usage data to create a unique identifier to track users.' It is said that it has been done. This is believed to be due to the 'transparency of app tracking' rule becoming more stringent in iOS 14.5, which is due out in the spring of 2021.

Analyst Eric Seufert reports that 'updates to apps that include the Adjust SDK, which collects data for device fingerprints, have been rejected.' The Adjust SDK is the SDK used by many apps, and Seufert estimates that more than 50,000 apps will be affected.

Per a number of developers, Apple has begun rejecting app updates that include the Adjust SDK related to its collection of data used for device fingerprinting.

— Eric Seufert (@eric_seufert) April 1, 2021