Earlier quoted context omitted.
You could have the logs signed with Slack's PGP key so they cannot be altered without the signature causing a mismatch.
Arguably, but what if the company wants to find proof of, say, two employees colluding to exfiltrate sensitive data or something like that? Would they have to convince them to turn in the PGP signed logs? More generally like the parent I don't see why a company couldn't have full control over their corporate tools.
In that case spying Slack usage is simply not enough: the employer should need to spy every single move every employee makes inside and outside the company, which of course it's not possible (well, except if the company is located in a fascist state).