Due to Teams/Channels the left sidebar menu on Stack Overflow is already a little different from the rest of the sites (with Meta Stack Exchange currently being the only available example). But a significant difference that sticks out is that SO doesn't seem to have an "Unanswered" menu item, while MSE does:
This seems unrelated to Teams/Channels and thus seems to be an inconsistency in a UI overhaul that strives to largely unify the experience on the various SE sites.