Apple removed access to the UDID because it was being used to target advertising and correlate users across different publishers -- basically privacy issues. Fo

Apple removed access to the UDID because it was being used to target advertising and correlate users across different publishers -- basically privacy issues. For a while there was a slew of third party solutions which involved hashes of MAC addresses. Some time thereafter Apple created IDFV and the companion, IDFA (For Advertiser) and really cracked down on cross-vendor correlation.