setPreferredBusArrangement removed in JUCE 4.3.0?

This is because in 4.3.0 they have introduced a new, revised multibus API.

The first attempt, based on setPreferredBusArrangement() was too complicated and didn’t cover all edge cases.

Until they update the official doc (which I hope is done soon), you can refer to the following topic for a description of how it works now: