Originally Posted by
DavidDTW
If you do not cancel in advance of the flight and are marked as a no show, the airline may suspend the ticket. If they do, you would lose all value on the ticket. It usually doesn't happen right away and you normally have a day or two to exchange, but it can happen anytime. That's why it is always better to cancel ahead of time.
I know DL at least temporarily was just automatically moving the unused ticket into your My Wallet. I'm not sure if that is still the case.