Udacity's classroom environment experience for students is always progressing with our new content - our quizzes, content types, and interactive workspaces environment versions are linked to the versions of our classroom content.
As we improve our features in the classroom, our older content versions eventually become incompatible and unsupportable, therefore it is not feasible to maintain them beyond the 12 months.
Although this means that we have to set limits on the time we host and support this classroom content for students, we do understand that students would like to reference these concepts later and so we allow students to download that content for personal use (forever), in accordance with our Terms of Use.