MIDI input is not a problem, it works rather well. 
The DAW translates MIDI events into either note-events or automation envelopes (so MIDI CCs are indistinguishable from parameter automation) for the plugin. The plugin provides a default mapping using IMidiMapping.
MIDI CC output is awkward, emitting kLegacyMidiCCOutEvent - so the DAW translates MIDI to automation, but you have to translate it back yourself.
(I personally still think we should be able to tag output parameters using something similar to IMidiMapping, and instruct DAWs to either translate back to MIDI or automatically hook up to any following IMidiMapping-tagged plugins as sample-accurate automation, as required.)
AFIAIU, the official stance is that MIDI output support in VST2 was kind of an accident, everyone should have just realised this and probably not used it (?), so we don’t get to be frustrated that VST3 removed it. The lack of widely-supported alternatives is our problem, not Steinberg’s responsibility (even I suspect alternatives are rare partly because VST2 covered it so well).