I think the bigger problem that the OP pointed at is that whatever adjectives you attach to this, surveillance is almost always the wrong way to change behavior, unless your desired outcome is to make everyone suspicious of everyone else.
What's stopping these folks from creating an out-of-company channel to do the bullying and attacking in coordination via that means, or tricking the victim into joining them in the new side channel?
The answer to bullying or shitty office behavior is not monitoring. That cover-your-ass because the real answer is hard. Improve your company culture. Fire people who are detrimental to the team. KNOW YOUR TEAM! So often I hear about these things and what you find is a shitty manager who has no idea how to be a manager and says "well they get their work done."
You realize this has far more to do with than workplace harassment, right? Have you never heard of employees using a competitors proprietary information? Or arranging fraudulent financial transactions to cover losses? Or discussing how to mislead investors or watchdog agencies? What do you expect this company to do when they are involved in a legal dispute like this, and have to explain to the court why they have company sanctioned, un-auditable computer systems/applications that helped their employees break serious state and national laws?
I suppose my experience in large corporations is limited. But I'm going to hazard a guess that for all but the biggest corps "company sanctioned" is a not an official term, and that most have given little-to-no thought about all the various ways they need to keep track of the way their employees communicate.
Keeping in context with the OP, Slack allowing admin access to all conversations is a cover-your-ass corporate move, not a solid new tool to combat workplace cultural issues. Perhaps once in a blue moon employee surveillance and bad culture might intersect and prove useful. But that should in no way be used to justify corporate surveillance.
As an elected government official, I understand the importance of papertrails and record-keeping. But the mere fact that so many companies USED SLACK WITHOUT THIS FEATURE, means most had no qualms about side channels being un-auditable before. And now this is just sweet sweet honey to corporate overlords.
What's stopping these folks from creating an out-of-company channel to do the bullying and attacking in coordination via that means, or tricking the victim into joining them in the new side channel?
The answer to bullying or shitty office behavior is not monitoring. That cover-your-ass because the real answer is hard. Improve your company culture. Fire people who are detrimental to the team. KNOW YOUR TEAM! So often I hear about these things and what you find is a shitty manager who has no idea how to be a manager and says "well they get their work done."