That's what I'm thinking. I mean... of course employers expect to be able to have access to communication done by employees in the course of doing their jobs. They've been able to do this with every (archivable) communication medium since the invention of writing. Where is the news here?
If you want privacy use a private channel. Your employer's work tools don't qualify.
By private channel, I assume you mean a completely separate system that is not slack, and not a private slack channel (which, prior to this, was private from slack workspace admins, with hilarious results).
If you want privacy use a private channel. Your employer's work tools don't qualify.