Apple explicitly says that apps can't require app tracking in order to use their app, they're removed from the app store if they do.
Keep in mind "app tracking" here means reading your device ID, called the 'Identity For Advertisers' (IDFA for short). They do this so that they can cross reference that IDFA with data that other apps have collected on that same IDFA, allowing them to connect the dots that it's the same user.
This app tracking transparency feature makes it so that they can't access your IDFA, and can only show you ads based on info you provide within their own app, not from the other apps you use. It keeps your data silo-ed to each app.
Seems Facebook could nonetheless offer two tiers of service for those with and those without tracking.
Although a bit orthogonal, Instagram will not let me post from a Desktop browser but will from a Mobile browser. (So I switch my User Agent to a phone on the Desktop when I navigate to Instagram and wish to post.)
Keep in mind "app tracking" here means reading your device ID, called the 'Identity For Advertisers' (IDFA for short). They do this so that they can cross reference that IDFA with data that other apps have collected on that same IDFA, allowing them to connect the dots that it's the same user.
This app tracking transparency feature makes it so that they can't access your IDFA, and can only show you ads based on info you provide within their own app, not from the other apps you use. It keeps your data silo-ed to each app.