How do I disable this red warning appearing in every new conversation in iMessage?
Turns out that this is Apple internal messaging, intended for employees.

It appears that, if the machine has a directory named AppleInternal in the root directory, certain Apple-employee-specific features of the OS — including the above one — are silently enabled, without explanation.



I've no idea how this happened to my machine, but I rm -rf'd /AppleInternal, rebooted Messages.app, and the message has stopped appearing.