I just tried out the backup with the template. But there seems to be no completed tasks.
Is a backup/template with completed tasks possible?
The premium version of Todoist does a backup of your active projects, notes and tasks daily which you can restore from and can be found on the Backups tab in Todoist.com settings.
The export feature is used mainly as a template tool, to allow easy creation on a new project.
Neither of these at the moment include completed tasks, but we will consider adding this in the future.
Is it possible to restore completed tasks that have been accidentally marked as complete?
Yes, it is. To do so, enter the project in which it was completed, click on the circular icon in the bottom-right corner and then un-check the task from there.
En anglais. Sniff