org-agenda-archives-mode

org-agenda-archives-mode is a variable defined in `org-agenda.el'.
Its value is

nil

Documentation:
Non-nil means the agenda will include archived items.
If this is the symbol `trees', trees in the selected agenda scope
that are marked with the ARCHIVE tag will be included anyway. When this is
t, also all archive files associated with the current selection of agenda
files will be included.