Has Apple implemented a Digital Leash between the Apple Watch and the iPhone?

Can you explain what you mean by "digital leash"?

The watch will show you an icon (read iPhone with a slash through it) at the top of the screen if it's out of range of the phone. There are no audible notifications.

If you swipe up to access glances, swipe to the glance that lets you turn sound and airplane mode on/off, you'll also see an icon of a phone. It you tap that, it will make your phone make a noise so that you can find it if you've mislaid it nearby.

I think the "leash" is intended to alert you that you have become separated from your iPhone. So when you accidentally walk away from your iPhone in a restaurant or at a party your watch vibrates to alert you. Then you can use the facilities you describe to locate your iPhone.

No, that is not a feature of the Apple Watch.

Submit your feedback to Apple requesting such a thing here:

http://www.apple.com/feedback