An Apple engineer explained the issue in a separate thread here: https://developer.apple.com/forums/thread/714467?answerId=734799022#734799022 Short summary is that if the warning stack trace shows SecTrustEvaluateIfNecessary and SecTrustCopyCertificateChain followed by WebCore and WebKit code, then it's a known Apple issue. Using WKWebView causes Xcode to show this warning, and there's nothing devs can do to fix it. But it's just a noisy warning, not causing any real problem. Apple plans to fix the warning at some point but no ETA. If you don't see the stack trace, you can expand the warning to view it by clicking the arrows ">" next to the purple warning icon in the left pane. More details on the specific issue and on this Xcode diagnostic feature in general are in the thread linked above.

Will this behavior be changed now that the IDFA is available on iOS 14 until early next year? Returning NO even when the IDFA is available/non-zero breaks compatibility with many existing apps and SDK's which don't use the IDFA unless the value of advertisingTrackingEnabled is YES.