Meh. Nobody should be surprised by any of this in the slightest. If your employer provides / pays for any kind of communications tool, the only sane position is to assume that they can - and probably do - monitor every single byte you send.
If you want privacy use a private channel. Your employer's work tools don't qualify.
There are a number of things I might mention to a coworker over a private IM which wouldn't necessarily put my employment at risk, but would be awkward for management to suddenly have access to.
A couple made up examples:
"I'm super sick, but $boss is really pushing me to get the report out. I just want to go home and be sick all alone."
"I hate management's decision to reduce vacation days. No wonder we can't keep people around here."
"Did you see Tom's email? It's kinda awkward that he thinks he's a strong contributor to the group..."
Never write something you wouldn’t want printed on the front page of a newspaper.
You're begging the question.
"Competent and trustworthy" people won't abuse their power by definition. Anyone who abuses their power intentionally is untrustworthy, and anyone who abuses their power unintentionally is incompetent.
In the real world there are many incompetent and untrustworthy leaders. Slack has no choice but to operate in the real world.
OTOH, I still think being paranoid is the safest policy, so if you're plotting to overthrow your boss, or sell secrets to your biggest competitor, I still wouldn't do it on the company networking, and/or using a company computer.
Cloak-and-dagger games are very similar to building your own crypto: likely to be broken at a fundamental level, never mind the amount of magic security glitter that you pile on top.
From the actual source: https://www.echr.coe.int/Documents/Press_Q_A_Barbulescu_ENG....