What is your expectation if the UI does not show that? That your communication is private? Why would you expect private communication when using your employer's infrastructure or, worse, a third party infrastructure like Slack?
No. Just feel like who owns the keys is relevant. I can see, for example, if my employer is using a self-signed cert within my browser. Though I understand they have other ways to see what I'm doing in a browser even if there is no MITM cert.
If you don't own the keys, it's not your data.