Thanks. I still doubt it though. Firstly even without the IDFA they can heuristically identify devices in vast majority of cases, in fact even more accurately compared to the IDFA method given the current opt-in rates of 30-50%. Branch does this with deep linking and it's pretty successful despite the tightened privacy on the iOS side (I can imagine how Branch and similar services irritate Apple).

My hypothesis is that Facebook pushes developers to enable IDFA because it saves them some effort: of course it's faster and easier than heuristics. Therefore, apps don't benefit from enabling the IDFA. But I might be terribly wrong and am open to counter-arguments.