I would personally do this by implementing means of saying Done in a Flow thread. The entire thread should not be marked as anything, only a specific message, as extra attention may still be needed to things a contributor forgot to do.
Topic on Talk:Structured Discussions
To be specific: if we implement means to mark an entire topic as something that's been done, it's very easy to miss some things. (You ask me to paint a fence and a door, I paint a fence and leave a Done note, everyone else sees a "DONE!" in topic header and doesn't bother reading.) I'd like to only see this feature in replies somewhere, and it might be hard to design it without clutter. But it's much much more needed than the current "it's fixed so let's close the entire topic and make it unreadable without unhiding" system.