According to some comments on the Unity forums:
One user said back in February, “UnityScript has already been abandoned. Just no one has told the marketing team yet.”
Then just today (at the time of this post), a Unity representative replied,
“UnityScript will continue to work in 2017.2. We removed the menu item because we would like to avoid new users picking it up, but you can still add new UnityScript files to your project by other means” (emphasis mine).
So there we have it. UnityScript is (likely) being removed and this is the first step: preventing new users from starting with it.