Deleting a Workspace in Trello is permanent and there is no undo.

However, the Boards within that Workspace won't be deleted, instead they will be closed when deleting a Workspace. You can find and reopen closed boards by visiting https://trello.com/my/boards.

So to recover these boards, you can create a new Workspace and the Boards will be in your closed boards section. See this link for more info: https://support.atlassian.com/trello/docs/deleting-a-workspace/