URLs embedded in emails from Apple Mail archived in DT not clickable?

That’s too bad - making it much less useful. You have better knowledge than me, but wouldn’t shouldOpenURL in QLPreviewControllerDelegate allow this or am I misinterpreting the QuickLook API / code?

/*
 * @abstract Invoked by the preview controller before trying to open an URL tapped in the preview.
 * @result Returns NO to prevent the preview controller from calling -[UIApplication openURL:] on url.
 * @discussion If not implemented, defaults is YES.
 */
- (BOOL)previewController:(QLPreviewController *)controller shouldOpenURL:(NSURL *)url forPreviewItem:(id <QLPreviewItem>)item;

Digging a bit deeper, there’s also some background on it here (Swift):

It’s quite common for links to be existing to the documents previewed by the Quick Look Preview Controller. When that happens, there might be cases where you would like the links to be working, but there are also cases where the links should trigger no action at all. The following delegate method exists for that reason exactly, as it’s the perfect place to apply your logic and allow or prevent links from opening the target URL(s).

This is an iOS-only controller.

Ah too bad - there’s no alternative to do the same on macOS / Cocoa?

Unfortunately not.