Versions, history and export
Every card keeps its revisions. The whole board is saved as versions you can restore.
Each card edit is a revision; open a card's history to compare any two. A nightly job saves a version of every board, and one is saved before sweeping changes such as find and replace. Admins restore a version from Versions.
Export a project as Markdown from the project menu. Your own data (profile, workspaces, cards you wrote) can be downloaded from Settings → Account, and the account deleted there too.