The Arduino PluggableDiscovery feature introduced in 1.8.9 enables to detect Arduino boards that are not connected by USB but are otherwise available through other channels (network, radio, etc.).
For now, it doesn't seem to work in the beta (or perhaps it is not implemented yet).
We discovered this bug with our own tool, which enables the discovery of Arduino compatible boards available over CAN-bus. Our logs show that the discovery module is not called by the Arduino IDE at all.