I’m doing this in 2 parts, because it’s about 2 issues that should be evaluated on their own merit. First, defining what we are actually talking about here:
“Obsolete Content”? We’re only talking about outdated answers here, aren’t we?
I can’t imagine questions getting outdated. Even if they are about old technology, they still apply to that. Even more, if someone lands on a question and they read “This problem doesn’t exist anymore, because now you can just do X”, that’s something you don’t want to take away!
Obsolete via software versions
Not a problem IMHO. Currently at SO, questions have “… in version X.Y.Z” at the end. Python, with two “big” versions (2 & 3), just has tags for these. Answers routinely say “If you’re using version X.Y, you can do […]. Otherwise, you’ll have to do […].”
The only (small) issue I see is if new versions provide an easier way to do something for an old question, which is currently handled with comments on SO.
Obsolete because of danger
This is probably the biggest and most important part: “Answers that are not safe anymore”.
However, I’d argue “anymore” is a stupid distinction to make. We need a way to tackle all “Answers that are not safe”. No matter how old.