Today, on apple mailing lists, someone said:
Keep in mind that out-of-date information never gets removed from StackOverflow. It's a treacherous place to use as your primary source of information.
I believe, we all come across such Q/A frequently. Updating them is a huge task. However, marking/signaling the answers as outdated could be useful.
It could be presented in the form of a visible message telling that the information might be outdated. The number of users that did report the post outdated could also appear. There may also be other ways to proceed. Just some thoughts.
For instance, for those who need to check Apple's documentation, APIs, sample code… they have such system. It is limited, but effective at telling some content is outdated.