I have spoken to a number of developers that are seeing higher-than-expected opt-in rates for ad tracking with their iOS14 beta apps (30%+). That's encouraging, but there are two reasons to leave the champagne corked: (1/X)

2/ There are some sampling bias problems with iOS14 early adopters. These people are the most likely to understand the benefits of ad tracking (and potentially to be developers themselves!). It's always difficult to contextualize early adopter data.

3/ And there's a two sided problem with opt-in: even when the advertiser has the IDFA for targeting, it needs publishers to also have it in order to target on those sites. Even if we see 50% average global opt-in, that's still only 25% IDFA targeting viability