What impact will this have on retargeting campaigns on iOS?

This will have a significant impact on retargeting providers. Web-based retargeting will still be possible (within the parameters of existing restrictions like ITP and user opt-in after install), but without IDFAs, we expect that in-app retargeting can no longer exist as it does today.

Additionally, SKAdNetwork does not support re-engagement conversions, so no attribution of any kind is possible if the user does not opt in via the ATT framework. We’re aware that this is a major gap: it’s clearly prohibited by the language of Apple’s ATT policy, and unfortunately not supported by any of Apple’s new functionality. We hope Apple will clarify their intentions here.