Site deaths are when sites go offline, taking content and permalinks with them, and breaking the web accordingly. Site deaths are one of the big reasons why you should own your own identity and content on the web.
This is a chronology of content hosting sites that have died, removing millions (billions?) of permalinks from the web. This is specifically for content hosting sites which permitted end-user posting. Not random sites nor pure “app” sites where there’s no loss of user data. If a game site goes down and takes down play history/scores, or if Gmail went away those would both represent loss of user data (though no public permalinks).