If email lists can’t be used for re-targeting due to iOS 14 ATT opt-out, what can still be used to re-target?

You basically cannot retarget if there is no ATT opt in. Retargeting requires knowing user level information to target again.

Assuming you mean re-targeting website users with ads on iOS apps, any match key type can be used (like emails), but only for users who opted in when presented with the ad hosting app's AppTrackingTransparency prompt. If the user did not opt in via the ATT prompt, or the ad-hosting app doesn't invoke the prompt, no data gathered from within the app can be used for cross site purposes like matching for targeting or measurement. This includes Facebook, Inc. ads, and Facebook has advised advertisers to expect custom audiences and Pixel/SDK sourced audiences to get smaller after ATT.