This is the answer. We don’t need a sidebar. It’s one extra click and keeps the forums index pretty clean. We get a lot of posts, and a recent posts feature wouldn’t really work.
As mentioned already, you can see the recent posts on the forum by clicking on the link "New Posts" at the top right of all pages here on the forum. The same page can also be found in the drop-down menu of "Members" (see image) along with a link to the "Recent Activity" area too.