Boards get removed after 2 months, regardless of activity. This is because of 2 reasons - 1 it automatically removes old/inactive boards from my db :). And 2 it keeps the boards/tasks more focused for about 4 sprints. I've received a lot of feedback about making the boards stay alive as a paid feature, but I'm not going to make something paid when you can just go to a competitor like Trello if you want persistent boards.