Just to add more details. The multiple channel configurations in Pro Tools appears only when I have the old deprecated field empty (so I'm not using any old code). By default now Pro Tools returns the pic in OP.
I've played a bit with what I could find in the forum and examples and apparently this code seems to returns just Mono and Stereo (as it should), but no Mono->Stereo.
bool setPreferredBusArrangement (bool isInputBus, int busIndex,
const AudioChannelSet& preferred) override
const int numChannels = preferred.size();
// do not allow disabling a bus
if (numChannels == 0) return false;
// always have the same channel layout on both input and output on the main bus
if (! AudioProcessor::setPreferredBusArrangement (! isInputBus, busIndex, preferred))
// you must ALWAYS call through to the base class if you accept the layout change
return AudioProcessor::setPreferredBusArrangement (isInputBus, busIndex, preferred);
I have no idea if this is the correct way to do what I need (a simple Mono, Stereo or even Mono->Stereo plugin).