gnus-boring-article-headers

gnus-boring-article-headers is a variable defined in `gnus-art.el'.
Its value is

(empty followup-to reply-to)

Documentation:
Headers that are only to be displayed if they have interesting data.
Possible values in this list are:

'empty Headers with no content.
'newsgroups Newsgroup identical to Gnus group.
'to-address To identical to To-address.
'to-list To identical to To-list.
'cc-list CC identical to To-list.
'followup-to Followup-to identical to Newsgroups.
'reply-to Reply-to identical to From.
'date Date less than four days old.
'long-to To and/or Cc longer than 1024 characters.
'many-to Multiple To and/or Cc.

You can customize this variable.