Any work tool is like that, including slack and teams. If you're using a corporate device or tool paid for/managed by your employer, you have no privacy whatsoever. If you're using the internet at work, IT knows at least which sites you visit
Usually the logs/conversations don't get read, they just have words that get flagged (from swear words to drugs to who knows what else), the rest is mainly in case something happens they can look into it more and maybe cover their ass.
That said, I bet more data goes to microsoft from teams than goes to slack from slack, so in that case I bet slack is a bit better