Quick answer: I2C8 is a MIDI plug-in. Ableton Live only shows audio plug-ins in the AU plug-in drop down menu.
Plug-in drop down menus in LiveI2C8 is a MIDI-effect plugin for chord sequence generation and sequence variant exploration, supporting the VST and AU standard.
Typically, the AU standard is used on a Mac, even for MIDI applications (especially in Apple Logic). This may cause users to think that Ableton Live also treats it the same, but this is not true - instead, Ableton Live only shows audio plug-ins in the AU plug-in drop down menu. Consequently, I2C8 is only shown in the VST plug-in drop down menu.
This is behavior specific to Ableton Live, if you're using a different DAW check its user manual to identify how this is handled by the manufacturer.