org-agenda-skip-deadline-if-done

org-agenda-skip-deadline-if-done is a variable defined in `org-agenda.el'.
Its value is

nil

Documentation:
Non-nil means don't show deadlines when the corresponding item is done.
When nil, the deadline is still shown and should give you a happy feeling.
This is relevant for the daily/weekly agenda. And it applied only to the
actually date of the deadline. Warnings about approaching and past-due
deadlines are always turned off when the item is DONE.

You can customize this variable.