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
I use Slack for personal projects and Teams for work. I think both are fine. The main reason it made sense to use Teams at work was because there were a number of products in use by different teams. IT had Slack and the rest had Zoom. Zoom was raising their costs and we already had Teams as part of 0365. So it was either buy Slack licenses for the entire company or just get everyone on Teams. It was kind of a no-brainer and it was hard to come up with a convincing argument to pay for Slack for everyone other than "Microsoft bad".
It’s funny you mention that about teams and O365. Microsoft just announced O/M365 licenses will be sold without Teams now, in the US. Something something antitrust lawsuit.