Apple has announced changes with iOS 14 that will affect how we receive and process conversion events from tools such as the Facebook pixel. Businesses that advertise mobile apps, as well as those that optimise, target and report on web conversion events from any of our business tools will be affected.
Specifically, Apple will begin to require that all apps in the App Store show a discouraging prompt to users on iOS 14 devices, in accordance with their AppTrackingTransparency framework. Apple's policy will prohibit certain data collection and sharing unless people opt in to tracking on iOS 14 devices via the prompt. As more people opt out of tracking on iOS 14 devices, ads personalisation and performance reporting will be limited for both app and web conversion events.
More about, read source : https://www.facebook.com/business/help/331612538028890?id=428636648170202