From the Release Notes:
Xcode 11.4 includes SDKs for iOS 13.4, iPadOS 13.4, tvOS 13.4, watchOS 6.2, and macOS Catalina 10.15.4.
Xcode 11.4 requires a Mac running macOS Catalina 10.15.2 or later.
I believe this means that when App Store validations start to fail and we start targeting new OSs in the coming months, the most successful troubleshooting method will be “install Catalina”, which is disappointing.
To distribute your macOS app as a universal purchase, specify the same bundle identifier as your iOS app in the Xcode template assistant when creating a new project.
Unfortunately the feature appears to rely on matching bundle IDs (which is problematic for some existing apps because changing the bundle ID creates a new app):
If you choose to build your Mac Catalyst app with a custom bundle identifier that does not match your iOS app, you will not be able to distribute the app as a universal purchase.
There are several usability improvements in both the Simulator GUI and command-line tool:
simctlnow supports modifying privacy permissions. You can modify privacy permissions to create known states for testing purposes.
- Simulator supports toggling appearance for iOS simulators (13.0 and later). From within the app select Debug > Toggle Appearance.
- Simulator now has a menu item and keyboard shortcut to bring up the app switcher in iOS Simulators.
simctl status_barnow allows changing the operator (carrier) name.
- Simulator supports simulating remote push notifications, including background content fetch notifications.
- Simulator has a new UI that streamlines working with simulated devices. Simulated device windows have a standard title bar, with buttons for common tasks. App-level settings are now available in the Preferences window.