While this can be done through other channels (in person or private cellphone) allowing it on corporate infrastructure without monitoring is not acceptable.
Unaffiliated? The company a customer that is paying Slack for a messaging service, that certainly isn't 'unaffiliated'.
If you've ever dealt with toxic work relationships, you know that stopping harassment isn't going to come down to how you are allowed to pass notes.
The only use I can see for this would be evidence after the fact. Surveillance is almost never the proper way to enforce acceptable standards of behavior.
Company policy for one could be applicable.
You’re missing the point though, what you do with non-company provided tools is held to a different standard from officially blessed and sanctioned ones.