Financial Markets collusion
If the sell-side banks take this up en masse, expect to see regulators and prosecutors rubbing their hands with glee in about 5yrs time.
Execution traders interact with other banks massively using IM. Including placing trades. In *1995* it seemed like over 50% of communication and 75% of wholesale/interbank trades in the FX market. Thing is, like email+socialmeeja, people become innured then immured to and in it, and start to treat it as a casual verbal conversation. And can reflexively put things in writing which formally record their legal culpability for pulling a swifty.
At the various legal busts for banks colluding on market manipulation, for example, (typically LIBOR or BBSW ref rates) the IM transcripts are simply laid out and are typically damning.
Well, Slack is IM+heroin.
Dodgy traders are that much more likely to stupidly put something on formal record.