Sometimes threads just fade out as the OP disappears for whatever reason. Sometimes threads are zapped. But the abandoned or unresolved still feature until they slip away in time. I quite like to be tidy and would prefer to Hide stuff I'm on that is clearly defunct/inactive. It can be a bit of a downer too seeing those situations left hanging.
It can be done by opening up the thread and hiding it from there, of course, but I'd prefer less faff. Plus this doesn't work when a thread is deleted. There's the remove button on "threads I'm watching" so that kind of arrangement but with a hide button would be cool? Might involve a new section like an active archive to house hidden threads?