The nuclear option to opt out of tracking on iOS

This feature has been around for many years, previously known as "Limit Ad Tracking".[1] They may have changed how it works under the hood.

Apple could have disabled cross-app sharing of Apple user identifiers by default, but iOS 14.5 instead has per-app permissions.

[1] https://en.m.wikipedia.org/wiki/Apple_ID_for_Advertisers

This may be locked by your mobile service provider, or corporate policy if you have a managed device.

Some report success after resetting their network settings via Settings > General > Reset.