I've checked the API on Big Sur 11.3 and Monterey beta within virtual machines. Looks like the API works fine, although there is a bug with NoteOn messages on Monterey. So I really don't understand what API Availability column means since API available beyond version limit.

Well, I found this project: https://github.com/phracker/MacOSX-SDKs. It contains all API headers for different macOS versions. Looking into 11.3 folder I see that MIDIInputPortCreate still there. But I don't know whether the function actually works or not.

Maybe someone can look into CoreMIDI.h file on macOS 11.3+ for example to see whether MIDIInputPortCreate function exists there or not? I would be very happy for the help!

OK, I finally figured out how to build loopback system with CoreMIDI. Source is an input device from an app point of view Destination is an output device from an app point of view So we just need to create destination with callback in which we will notify sources (with MIDIReceived) about new MIDI data arrived. So we provide source reference as an argument for callback in MIDIDestinationCreate and use this source inside the callback.