I didn't know it was happening already, but I believe that's one of the changes to AdSense that Google announced back in April -- that they're no longer offering custom channels for ad units (when you create an ad unit). See this page for more: [support.google.com...]
Adsense engaged into a process of simplifying to the maximum the system. More advanced users, can certainly relay on GA to get more detailed information about ads stuff ... I guess, because I never used GA, but this is what I understood from reading at others' messages.