This is a great question and I just debugged this myself. Here's what's different in iOS 12.2: The UIDragInteraction is only added _after_ the WKContentView is added to a _window_ (not just a superview).
Here's how I found it:
I searched through the WebKit source and found this: https://github.com/WebKit/webkit/blob/65619d485251a3ffd87b48ab29b342956f3dcdc7/Source/WebKit/UIProcess/ios/WKContentViewInteraction.mm#L4953
That's the method that creates and adds the UIDragInteraction. It turns out that this method (setupDataInteractionDelegates) actually exists on WKContentView. So I set a symbolic breakpoint on "-[WKContentView setupDataInteractionDelegates]". Sure enough that breakpoint was hit. I then displayed the backtrace in lldb:
(lldb) bt * thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 50.1 * frame #0: 0x00000001115b726c WebKit`-[WKContentView(WKInteraction) setupDataInteractionDelegates] frame #1: 0x00000001115a8852 WebKit`-[WKContentView(WKInteraction) setupInteraction] + 1026 frame #2: 0x00000001115a5155 WebKit`-[WKContentView didMoveToWindow] + 79
So we see that the interactions are only added AFTER the view is added to a view hierarchy that is attached to a window. Your old code should work, as long as you only call it after the WKWebView has been added to a window.