It depends. It is much harder to get some SEO value fo forum topics. It looks like Google values heavily the very first post, not following answers. But the first one quite often is not that… good in the meaning what people are searching for.
Sure, something like ServerFault, StackOverflow etc has high value, but those are so big. And even then I’ll get more hits to more bloggish sites.
But even here is a lot of topics with zero SEO value, because those aren’t reflecting asked questions on side of Google.
My forum is really small, but I’m using it quite often similar way than WordPress. I had few diary type topics, but that didn’t work SEO wise. When I started totally new topic everytime I had longer explanation, or story, I got a way better response from Google.
How often old topics get views here? Not too often and that’s why we have autobump — and in most of time bumping fails because three years old topics are more or less so outdated.
If we stay in SEO value this will happend
- an visitor did google search and got a topic here
- that visitor read, doesn’t get an aswer and bounces
- visitor will continue to next hit
- google sees it as un-helpful hit
Please, don’t get me wrong. That is an universal question, not Discourse dependent. But it means one or two things: content itself doesn’t give anyking SEO value and most of content in the Net is dead weight.
What we should do in the perfect world would review old content every year, save useful ones and delete every topics and posts without any extravalue. Plus everything that have been unvisited.
Same thing what we should do with private files and images on our devices. But we don’t do cleaning, but we will buy more and more space.
No one will do it. Way too much work. Quite few would review content even if system would ask shall this and that saved or deleted. That’s why there is a huge risk where high value content will be erased just because of some time limit.
So — that poor admin will pay extra every month for storing old, rusted and dusted content. But it has nothing to do with SEO. SEO means only for content that is used, someway.