Steps to reproduce:
1. Create an empty project
2. Build to iOS & open in Xcode
3. Set up the project for uploading to iTunes Connect (Display name, bundle identifier, Capabilities etc.)
4. In top menu, click Product->Archive
5. Once building is done, click "Upload to App Store..."
6. Follow through the upload wizard with default steps

Result: Warning "Missing Marketing Icon - iOS Apps must include a 1024x1024px Marketing Icon in PNG format. Apps that do not include the Marketing Icon cannot be submitted for App Review or Beta App Review." is thrown.

Note: There is no 1024x1024 Marketing icon in Unity's Player Settings.

Workaround:
1. Under the project in Xcode navigate to "Unity-iPhone -> Images.xcassets->AppIcon"
2. Scroll to the bottom
3. Add a 1024x1024 icon
4. Proceed with reproduction steps 4-6