Verified tasks will be automatically archived (and disappear from plan) after 7 days. If your tasks are building up and you'd like to archive them more frequently, you can change the timeframe to 1 or 3 days within your project settings. Alternatively, you can change this to 30 days if you'd like to view archived tasks for a longer period.
If you ever need to refer back to an archived task, they can be displayed on the task dashboard at any time.
Selecting an Archive Timeframe on Web: