Steps to Reproduce

1. From VSCode use the Flutter: New Project command.
2. Enter myapp for the project name, let Flutter build the project files
3. Use XCode to open the project
4. Select the top level Runner node in the project tree view
5. Change the Deployment Target to iOS 12.1
6. Use CMD+B to build

The following warnings are generated:

Please update the deprecated code.

As of April 2018 all new apps submitted to the App Store are required to target iOS 11, (requiring XCode 9), come March 2019 all new apps submitted to the App Store are are required to target iOS 12.1, (requiring XCode 10)