While Apple are limiting access to IDFA for apps that don't ask for permission to track i.e. it will be all zeros they are also changing App Store policy. Specifically you are not allowed to track users using identifiers such as user ids, fingeprint ids etc if they have said that they don't want to be tracked. Sure Apple doens't have technical control over this but we know that they are willing to wield the ban hammer.

It'll be really interesting to see this play out. The experiment with the "Do Not Track"[0] header on the web failed becaused it was largely ignored by everyone. With Apple's tight control over the App Store you simply cannot ignore the rules because Apple can outright ban you for it.

0: https://en.wikipedia.org/wiki/Do_Not_Track

Prob more a combination of IP address, user agent, iPhone model, plus prob some other device fingerprinting identifiers these ad trackers found