The mailman setup on gae was slightly broken in such a way that it did not process bounces. I'm not sure how long it's been that way, but I suspect quite a long time. Some of the list admins might have noticed that a large amount of people were unsubscribed a couple of days ago. This is just a backlog of people who should have been unsubscribed already, some of them years ago, so no need to worry.

– tfheen