What is Android's 'userdebug' build variant and why is it considered insecure for production deployments?

• A.userdebug disables the Keystore and may substitute a software provider for it, and every key an app generates on such a build sits on the filesystem rather than in hardware
• B.userdebug enables root access via 'adb root', includes debugging symbols, and may run with SELinux in permissive mode - it is intended only for testing
• C.userdebug is signed with a publicly known test key, and anyone may build an update package the device will accept, with every signature check passing against that published key
• D.userdebug removes Play Protect from the image, and a system app may be sideloaded with no verification at all, typically leaving the platform to accept every package it is offered