Completed tasks disappear from projects
New user, so UE a distinct possibility. Can't find explanation in prior dialog.
Started loading projects into ToDoist. Nice product; but I need to keep track of those things that have been completed. When I check them off, they entirely disappear.
That's an intended behavior for parent tasks. Only sub-tasks will remain on the list until their parent task is completed.
However, you can access these tasks later by entering the project in which they were completed and clicking on the circular icon in the bottom-right corner.
Additionally, with Todoist Premium, you could keep track of all completed tasks from all projects at once, filter them by date, project, label and visualize your productivity through the Karma score -> View All Completed option in the top-right corner.
Aha! User Error, as suspected. Thank you for the quick bailout.