The current Fly Delta app for Windows Phone has been officially retired. If you open the app now, you'll be greeted with a message that notes it is no longer supported, instead recommending that users book their flights via Delta's mobile website instead.

The message reads:

We're sorry for the inconvenience as this app is no longer supported. You can always visit delta.com from your mobile phone to book a flight, check in and view flight status.

Curiously, below that message is a button that reads "Update Now." However, tapping the button simply takes you back to the Windows Store to the same Delta link. While the app still remains in the store and available for download, it is delisted, so you require a direct link to access it.

Given the presence of that curious "Update Now" button, it's possible that Delta may be working on a UWP replacement app, but it's impossible to say for certain. In the meantime, you'll have to resort to booking flights on Delta's mobile site while on the go.

Thanks to k1s23 for the tip!