Let's clarify a bit, this is for the employer owned Slack workspace. If you have the client and have your employer workspace and then another random workspace that is not owned by your employer, then they can only see the messages on the workspace owned by the company.
And this is meant for backups really, it's not going to be easy to just follow random conversations of yours on a daily basis. If they want to go back and dig up some dirt they can though.
That said, if you are worried about it, and working on a computer owned by your employer, you should just assume everything you do is logged. Because some do that.
It seems like this requires Slack Plus and not just the Standard plan. It's not clear to me if you can access all historical data immediately after upgrading to Plus, though.
And this is meant for backups really, it's not going to be easy to just follow random conversations of yours on a daily basis. If they want to go back and dig up some dirt they can though.
That said, if you are worried about it, and working on a computer owned by your employer, you should just assume everything you do is logged. Because some do that.