She didn’t do anything to violate the CoC.
Given leaked transcripts some have seen and been unable to identify CoC violations, and many mods here from the TL also not being able to identify any clear violations, and no one being able to come up with anything that was not either customary in the TL or "pre-thoughtcrime" concerns of "maybe she won't follow the future CoC", the clear reason the “CoC violation” has not been shared is that there was not one. The claim that there was a violation is simply cover for getting rid of a voice bold enough to disagree with an SE employee’s plans.
And of course when threatened with legal action they released a statement saying "whoops, it was a mistake." (A mistake they took unilateral and extreme action on, refused feedback, and refused to undo went unsaid.)
In the end the only CoC that applies to this situation is the whim of people like Sara and David. They see no need to justify their removal of a voice they found annoying with anyone here; their contempt for meta, mods, CMs, and the larger community has been made amply clear through many communications and actions over these months in late 2019 and early 2020.
If anyone saw Monica clearly violate the CoC, feel free to say so; you don’t have to share the secret TL details. But no one has, so the obvious answer is that it did not happen.
I don't know why it's not obvious to people in the world that when people make claims that lack specific details, proof, witnesses, or substantiation of any sort that it's just a garden variety lie, but for some reason people have trouble getting their heads around that. But given not a shred of evidence from any of these sources, it's clear that the claims that Monica violated the CoC is simply a convenient ginned up statement designed to provide a veneer of legitimacy to an otherwise arbitrary and dictatorial move, to fragment opposition and ablate the effects of their actions somewhat in the community. Because of course you can't prove a negative, so the deck is already stacked against a claim of innocence (except for those of us that live in places that believe in "innocent until proven guilty").