It is quite clear that this feature is still a work in progress and it may never see the light of day.
Even tho this is nothing new, Android has never supported this naively. As seen from this report by XDA, Android P’s SystemUI APK has a piece of code that can launch predefined apps when a certain accessory is connected.
However, it is not cleat just how configurable this is, and which parameters you can tweak. And also, the whole context is not clearly defined. “Connected” could be for Wi-Fi, Bluetooh, USB…
Thing is, this is nothing new. You can do this on any Android phone with apps like Tasker. While it is most unlikely that this new Android feature will replace apps like Tasker, and even assuming it actually becomes a thing, it will still be a convenient thing to have.