I have just had to update two of my answers (Should I use #include in headers? and 'Self-sufficient header files in C and C++?) because the cross-referenced web site at GSFC no longer works. I wonder how many other questions have similar link-rot problems and (more particularly) what can be done about it.
I don't know what's feasible, especially given the size of the overall question and answer base, but it might be nice to have some periodic sweep process (maybe over a period of 3 months or so to keep the workload spread over time) that validates URLs and notifies the posters (questions, answers, comments) if the URL goes AWOL (404), or whatever.
I fear that otherwise good answers to questions become less relevant over time simply because the external resources to which they refer have changed.