When you start a new iOS project with the target version being <= 13 you will automatically be opted-in to using scenes. While this is definitely the way to go in the future you might want to manually opt-out of using scenes.
How to opt-out
First you have to remove the "Application Scene Manifast" entry from Info.plist.
Next, remove the scene delegate class if there is one.
Then, remove all scene related delegate methods in your app delegate. Those methods will have the word scene in their names.
Also in your app delegate, add
var window: UIWindow? if it's not already there.
That's it. If you've correctly followed these steps, your app should now work without using scenes, reverting back to the old lifecycle.