This is somewhat the opposite to this question, though it seems like it's the result of either a technical issue or a recent update. I restarted my machine today, and now when Slack comes up (via Windows desktop application), it shows the little toolbar / menu bar at the top, with the options to upper-left and the caption "Slack - [team name]". This isn't desired, and I would like to hide it again.
However there doesn't seem to be an option for it, whether looking around in Preferences or trying to use the answer in the linked question. Reinstalling doesn't help. Resetting the state of the app doesn't help. Googling it leads to plenty of links for people trying to hide or show the sidebar, but not much with regards to the menu bar.
How do you get rid of it? Is this now required by Slack? As a side note, it's now also running off the edge of one monitor when maximizing the window.