Hmm, I'm not sure if I agree with your proposal. For example, I wonder who will decide if a thread is 'informative or important'. Threads which you find important, might not be that informative/important according to other members. Furthermore, when will these threads be closed? As in: do you close it a day after the last post, a week, a month, etc.? For example, if you close a thread to early, you might prevent members from responding with meaningful responses, but you run the risk of getting 'irrelevant discussions' in those threads.
Don't get me wrong, I also often am looking for certain threads which I read/posted in a long time ago, but can't find them. However, I think that, for instance, a list of 'watched/marked threads' or 'threads to which you at some point replied' might be a more suitable solution to this problem.
