Yeah, I couldn't find a way to exclude pages on Chrome extensions. But I haven't thought about how I may identify dead pages from code. I have an idea I'll try out soon.
If it works, the extension will be updated automatically.
Pages that are dead/not interesting will stop appearing after a month, but interesting pages will linger on. It was important for me to keep the element of serendipity, so I can still find conversations that were once interesting.
Perhaps you shouldn't display anything older than 3 months, as after 3 months one can re-submit the page.