Word of warning: an AUv3 with type kAudioUnitType_MIDIProcessor (that is to say, an aumf, as opposed to an aufx) won’t pass Review. While aumf is a recognized type in iOS, and some hosts can see it, Garageband ignores aumf AUs, and that is the principle test host the reviewers use in the final step. It will past TestFlight, which is largely automated, but not actual Review.
Until they change this, you need to render your MIDI effect as an instrument or aufx (leave the “MIDI input” box in ProJucer unchecked for effects.) Bram touches on this in his document, and it has not changed. You also need to target iOS 11. Note that it will work fine. AUv3 passes MIDI through, and if you have a Midi Message handler in your process block, it will work as normal whether or not the MIDI input is “activated” in ProJucer.