First, your example question (“What’s the best mouse for programming?”) seems particularly bad since it is quite subjective and thus already discouraged here, on SO and on many other SE sites (with exceptions like the programmers SE).
Second, I am having trouble understanding exactly what you mean by “the structure of Unix changes over a period of time by default”. Of course individual Unix systems and their individual components (programs, tools, languages) evolve and change. Usually things do not change so much that “old” answers are completely inapplicable; there may be new options or tools but usually old solutions are still usable (though they may require small modifications).
Do you have any specific examples of “stale” questions/answers that are not already explicitly discouraged?
What would be the difference between expired/archived questions and normal questions?
- Would this new state just be another version of “closed”?
- Are you suggesting (an optional) mechanism for de-emphasizing old questions in search results?
Overall, I am not sure I think such a mechanism is necessary. I have occasionally run into “stale” answers on SO; usually I add a comment or a new answer if there is a better way to accomplish what the asker needed.
Maybe the situation is different for someone that is more focused on finding answers instead of giving them. An alternative might the ability to bump a question that has no “fresh” answers so that it might get some new attention; the system already does this for questions without an accepted answer, this alternative could be a way for users to manually bump questions that do not have “fresh” answers even if there is a (presumably old and stale) accepted answer.