Do we have an official SFF or SE policy/guideline on which questions (among closed ones) merit a deletion? I usually just exercise my judgement (e.g. duplicates are OK to stay to help with SEO, whereas obvious SPAM gets deleted), but would be nice to know if something formalized exists.
There is a general Stack Exchange policy, expressed in two of Jeff Atwood's blog posts:
But a closed question is no longer alive in any meaningful way, and certainly well on its way to the bit-bucket of /dev/null.
Let me be clear: we do not seek out deletion, by any means. But we believe not having the guts to cull some of your worst content is much, much more dangerous to your community than letting it sit around forever in the vague hope that it will magically get better over time.
The general guideline is that duplicates are not deleted (because they serve a useful function as search fodder) and other closed questions are deleted if there is no chance for them to be reopened (because there is no life left in them).