[bradknowles]

NB: I am not a lawyer, and nothing that is said here can be construed as to be legal advice. If you're really concerned, you need to speak directly to a lawyer who is admitted and recognized by the appropriate bar association for the jurisdiction in question, etc....

That said, the company owns that Slack instance. They can do whatever they want with it.

Now, it might be unethical for someone else to post as another person, but I'm pretty sure it wouldn't be illegal.

Slack does make it difficult for anyone to remove a post made by someone else. That is intentional. But if you can log into their account as them, then I guess they can do whatever they want as the person in question.

Disclaimers: IANAL, YMMV, etc....