I am missing very much the possibility to keep historical data of the accomplished tasks in order to
1) track when a specific task was finished
2) keep track of finished projects
3) export to a spreadsheet for example to analyze past effectiveness and compare tasks/projects.
Thanks in advance for your support,
All these options are available on Todoist except for the last one - you can't export this data.
To view what tasks were finished at what dates and times, please click on the Karma score in the top-right corner and then on the "View All Completed" link. This will show you a complete log of each task completion incl. completions of recurring tasks and allow you to filter it by time, label and project.
When you "complete" a project, you can right-click on it and choose "Archive" as there is no direct option to actually check it off as a task. After you do this, you can access it in the future anytime, simply by clicking on the "Show archived project" link below the project list.
As for exporting, although that's not possible yet, while you're on the "View All Completed" list, you can click the "Visualize" link at the top to view graphs illustrating your productivity per day or projects.