Which iOS versions include the “allow apps to request to track” feature?

The App Tracking Transparency (ATT) framework is available (and required) for iOS 14.0 and above users.

In terms of functionality though, only on 14.5 and above ATT is required to have access to IDFA, without prompting the user for permission to track the IDFA value will be set to 00000000-0000-0000-0000-000000000000.

Prior to 14.5, IDFA will be either available if prompted for ATT and granted permission by the user, or if a user is NOT prompted and the user has the "AAtRtT" toggle ON (which is the case for users that migrated from previous OS versions with a LAT OFF state).

Out of  the 4 available ATT states (Authorized, Denied, Restricted,notDetermined), IDFA will be available for Authorized and notDetermined, while zeroed out for Denied and Restricted.

This also would be the main reason why developers decided not to prompt for ATT prior to 14.5 in order not to maximize the IDFA coverage.