How to see all past (overdue) todos?

NotePlan can show you all overdue todos (in the past and open) using the 'Filters' view. To open it, click in the left sidebar on "Filters" near the top. Then choose the filter "Overdue":

Clicking on "Edit", you can modify the filter. By default, it shows you all calendar notes and scheduled tasks that are open and in the past. You can modify it to show you only the last month or last week by clicking on the dropdown right of "Timeframe". Or you can filter scheduled tasks which are found in the notes by turning "Regular Notes" on or off.