I'm running Xcode 16.3.
Below are the instructions from TN3187: To configure your Info.plist for scene support, you should add a UIApplicationSceneManifest key with a scene configuration:
Open your Xcode project.
Select your app target.
Navigate to the General settings for your app target.
Enable the “Supports multiple windows” checkbox in the Deployment Info section. There is no checkbox - just an arrow that goes to the info tab
Edit the Info.plist file and add a UIApplicationSceneManifest key [example follows] Where is the Info.plist file and how do I edit it? - none shown
So, the problem really came down to the fact that my projects were created a LOOOOONG time ago (2010), so they didn't have the needed plist items above. For anyone interested in supporting UIKIT scene-based life cycle, my recommendation is to generate at new project that is an "app" from the templates and copy over the SceneDelegate files into your project, copy the contents of the AppDelegate into your existing app delegate, and add the Application Scene Manifest plist item (+ sign) and edit them to match what the sample project has.