The change in question concerns something called the IDFA, which is the IDentifier For Advertisers. This is a random identifier that Apple assigns to its devices. This identifier can then be given to advertisers so that they can track the effectiveness of their adverts. It protects a user's privacy because, although the advertiser can see a particular, anonymous user has responded to an advert in a certain way, they don't get any personal details about that user from the IDFA. It allows them build up patterns of behaviour.
The change Apple will be implementing is that, in iOS 14, its devices will ask the user for permission to use the IDFA. Facebook (and other advertising platforms) fear that a lot of users won't give that permission and therefore advertisers won't be able to track the performance of their adverts effectively.
Like all ad networks on iOS 14, advertiser ability to accurately target and measure their campaigns on Audience Network will be impacted, and as a result publishers should expect their ability to effectively monetise on Audience Network to decrease.
Ultimately, despite our best efforts, Apple’s updates may render Audience Network so ineffective on iOS 14 that it may not make sense to offer it on iOS 14.
I'm afraid I have no sympathy for advertisers. Facebook's comments demonstrate they were quite happy to take this identifier knowing full well that users might not like them doing so.
I appreciate that advertising is crucial to the economy of lots of websites, but any tracking of a user should only be performed with that user's informed permission. If a site is worth it, I'll let them track me. If they want to deny me access if I won't allow tracking then that's fine by me, but I want to make an informed decision either way.
It strikes me as odd that the sort of tracking we take for granted these days was ever allowed in the first place.