Correct implementation of popups blocking functionality is not possible without using -[WKNavigationAction _isUserInitiated].
WKNavigationAction passed to webView:createWebViewWithConfiguration:forNavigationAction:windowFeatures: delegate method does not say whether or not the window was open by a user-gesture. When delegate method is called a user may not be touching the view anymore.
Popups blocking is a feature supported by every modern browser.
Another case where _isUserInitiated information is useful is preventing web adds from opening AppStore apps from a Web Browser.
Someone from Apple dev relationship requested we file a new radar. So we filed a new rdar://50095482