Yes, it does. This has the feel of "great new idea, nice in theory", spoiled by backwards compatibility and divergent implementations in practice.
Also the "not in OMNI mode" caveat at least has some justification.
From the MIDI 1.0 spec (my emphasis):
So it is efficient if implemented but the safe and secure method, assuming you have a list of notes turned on, is (sadly) sending all the corresponding Note-Off messages.
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The voltage divider on MISO is a test, not a solution. What voltage did you measure?
If your SPI slave devices do not tristate correctly, you will need to add a tristate buffer such as SN74AHCT125. You can then...
> I have a bunch of TL074 quad op-amps around, so I would use one of them since it works with 5V just as well.
No, it doesn't. TL072 and TL074 do not work well a couple of volts away from the rails, which is fine...