In environments like this, my trusted colleagues and I communicated using Signal (and before that, WhatsApp).
One somewhat paranoid department that was convinced they were being spied on (they weren’t; I saw the Slack admin dashboard and management was too cheap to pay for the retention and spying features) maintained the use of an ancient Jabber based group chat for their own internal communications.
This was around 8 years ago, but there was no MDM installed on our cell phones, regardless of if BYOB or company paid for device.
The only restriction was if you went to China, you took a burner phone (one of the old company phones, usually) and weren’t supposed to ever use it again once you left. I think they just sold them to a liquidator.
I guess that's a fair point. It cuts both ways, but given that so many people use Slack as opposed to talking, the exact words people used and when are could be open to view. Whereas, before all of this, you may only just have the minutes of any official meetings. Any side chatter not in the meeting room and/or exact phrasings would be lost to time.
its sold more as a way to store and all conversations than the ability to be a messaging application.
the original pitch was to make all information, even private conversation of previous employees, searchable.