I wonder if someone else thinks this needs to be taken up by the Core team.
There are a lot of top communities using the previous UI (with no sidebar) on Discourse because of that.
I found the sidebar super useful to organize large communities like ours, but the current behavior on Discourse is the opposite.
It feels like non-technical folks just don’t know how to use our forums because of small but significant details like this one.
There is always room to improve 
Meanwhile I’m testing this Theme Component: