Lots of links are broken. If you go to https://stackoverflow.com/questions/tagged/plone?page=188&sort=newest&pagesize=15 for example and try to go to each question and each link provided by the users you would be surprised.
If you're really interested in collecting all possible broken links, maybe the stackexchange api will be nice. Get the ids from all questions tagged Plone in https://api.stackexchange.com/docs/advanced-search#order=desc&sort=activity&tagged=plone&filter=default&site=stackoverflow&run=true, use these ids to get the answers in https://api.stackexchange.com/docs/answers-on-questions and the comments https://api.stackexchange.com/docs/comments-on-answers and then extract all links from all answers and comments and check which one of them is broken.
The https://dev.plone.org/plone/ticket/6805 are now all broken, see Are we still using https://dev.plone.org/timeline? for an explanation.