This question popped up in the close queue this evening. I'm all for closing questions that don't fit the scope of the site, but what do we really gain here? The question is four years old, has an accepted answer, and was probably only dug up by a badge hunter or new user that has no idea how to use the site. At what point do we leave well enough alone, know that the question was useful at some point, and realize that the community has evolved since? What about cases where technology changes? All questions and answers will eventually become obsolete. How do we handle this as a repository for Q&A? The only option seems to be regular review and editing, but how is that practical?
One thing that's gained is to make things less confusing for new users. If I was new to the site and found that question while searching with seven upvotes I might conlude it was OK to post my question asking for a recommendation for a RS485 to USB adapter. The What types of questions should I avoid asking? page in the new help center doesn't make it all that clear that shopping / product recommendations are off-topic (maybe that needs addressing).
Here's a recent example where the person asking the question has referenced a similar question that wasn't closed at that time:
The other thing is that closed != deleted so the information still remains available. I know that on Stack Overflow some old questions are automatically deleted, I'm not sure if the same script is running on EE.SE but even if so that wouldn't be deleted. You can see the rules at How does deleting work? What can cause a post to be deleted, and what does that actually mean? What are the criteria for deletion?
I think obsolescence is a bit of a different issue and those questions and related answers can stay useful for a long time. I've seen quite a few interesting hobbyist questions lately about parts not manufactured for quite some time but still available in limited supply. On a professional level understanding how old technology worked can be useful when updating an old design.
When a technology becomes obsolete one way to address it is adding new answers to technology neutral questions recommending a newer solution. Many could also be addressed by a new question, a future example might be "I read this question about controlling a relay and all the answers recommend a MOSFET. Now they are obsolete how do I control a relay with an XFET?".
The thread in question got dug up by the spammer, who had made a promotional post. That post was duly marked as spam (I have contributed to that). Then I've initiated close-voting on the whole thread, because it's providing a bad example and attracting spam too.
I've come across a few old questions, which were marked with something like the following:
sub-standard question, but we keep it for historical reasons as a relic from the early days of this stack
I'd like to post a link to such old question, but I can't seem to find one at the moment.