One word. Breadcrumbs.
This isn’t too much of a problem now but as time goes by, the forum board will most likely expand to meet requirements to keep things in the right place (see any forum board ever).
There is a tonne of whitespace on both the left & right that could easily be used as some sort of forum navigation that could possibly be implemented. Also, breadcrumbs are only 1 line high so it’s not as if it’d be in the way of any content.
Proposed Look:
Not a bad idea if you ask me, although it looks a bit distracting. 
You do know that the top bar has this right?
I don’t see the need for the side bar to be filled up with anything. But that’s just what I think
There isn’t whitespace for all of us…
That’s the category the post was filed under. Not breadcrumbs.
Didn’t actually think about this. Looking at your picture though, there is whitespace on the right side so you are able to push the content over and at the nav on the left.
The whitespace is part of the responsive design. Smaller screens see the same thing as a larger screen, but you end up with more white space on a larger screen. Putting something on either side would effectively mean you need a large screen to see it, or it would break the UI for everyone. No thanks.
And breadcrumbs are already on. You can click your way up the boards. Someone above already pointed it out.
It’s not hard to add a navigation to a responsive theme… Really easy actually.
As for breadcrumbs, as far as I can tell, that’s just a category (as I already mentioned).
You can have so that when user has a bigger screen, it can show more menus and when having a smaller screen (like mobile), less menus.
Youtube does this. Not commenting on the idea itself.
Okay so let’s break this down…
That is the “Community” Breadcrumb. (takes you to https://forums.spongepowered.org/)
This is the category in which it was posted,
And finally the topic link.
Does this not already achieve what you are suggesting?
EDIT:
This image shows it works with sub-forums.
Yes, I never said that this isn’t fine at the moment but if there are more levels added, this becomes a problem. I just wanted to bring up this as a problem and just hope the developers know what they are doing.
@Pink__Slime I’m failing to understand what you are tying to say. But if I can assume that I understand you for a moment, you are saying that if they add another sub-forum to say Plugin Development it will cause problems? If so, I fail to see why a problem would occur. To be honest I barely know forums to go any further than Forum > Sub-Forum > Sub-Sub-Forum.
If you would please give us an example, that would be amazing! Thank you 