I'm wondering what ID they use? If its identifier for vendor[0] then Segment (or any other analytics providers) would have more trouble connecting the dots to figure out who the person is. This is because per vendor, the ID changes.

If its the advertiser identifier[1], they can track across apps, however this is an easy fix by enabling "Limit Ad Tracking" in settings, which causes the ID to be all 0s.

0: https://developer.apple.com/documentation/uikit/uidevice/162... 1: https://developer.apple.com/documentation/adsupport/asidenti...