Display tasks which were completed within the X last days only
It would be good for the mind and for the morale, for a given view or filter, to keep track of the completed tasks
But this should not be leading to cluttered lists.
One possibility would be to display, in addition to the "active" tasks, only those completed tasks (with a nice strikethrough, good for the morale) which were completed during the last day, couple of days or whatever, according to your settings/parameters.
It would be helpful for me to see all my completed work tasks during the most recent pay period.