VST2 was officially deprecated last year by Steinberg. The majority of plugins at this point are still in VST2 format.

You missed the tuning field in the struct, which effectively makes it use 12TET only as a reference, not as the only tuning standard.

Perhaps you missed my original comment where I explicitly referred to using a 12TET pitch + offset? What you claim I missed is literally what I said.

I read through the MIDI 1.0 spec last night, and I was very surprised to see it said nothing about default note number to pitch mappings, other than that 60 is middle C. The only time frequencies seem to come up is for the (optionally implemented) messages to retune individual notes! (I'm not sure I have a copy of the whole thing, so I could be wrong.)

From what you say, it seems like the VST2 spec sets a standard scale to fill in this gap, which is interesting. It certainly would help with interoperability.

Anyway, I learned something from the clarifications arising from each of you correcting each other :-) (Though it's probably not worth continuing discussing whether there's any difference at all between "deals in 12-TET" and "uses 12-TET only as a reference" since you both seem to mean the same thing by these by now.)

MIDI doesn't explicitly mandate 12-TET, but attempting to do anything more complicated than a mild re-tuning of a 12 note-per-octave scale usually results in a poor user experience. For instance, it's no longer plug-and-play because the instrument and the synthesizer need to agree on a what each note means, and there isn't a way to communicate that. (Well, technically there is: the Midi tuning standard, but it's rarely implemented.) There's also the problem of the number of notes: 128 often isn't enough.

A standard workaround if you want to do microtonal MIDI is the one note per channel trick: each simultaneously-sounding voice is on its own channel, which can be independently pitch-bent.

For instance, if you want to play a note 10 cents above middle C, you pick a MIDI channel you're not using right now, send a pitch bend command to raise it by 10 cents, then send a note-on for note 60.

This gives you 16-voice polyphony (because MIDI has 16 channels). The downsides are that this only works on multitimbral synths (i.e. it works with most "romplers" but very few analog synths), and you need a convenient way to set all the channels parameters the same.

The one note per channel trick can also enable individual note volume swells by using channel volume. One note per channel has been standardized recently in MIDI as MPE.

There's also MIDI 2.0 now, which has per-note pitch bend among other features. Time will tell if it actually gets widely adopted, and whether synths actually implement the full spec or if they leave out features that only a small minority of their users care about.