gnus-inhibit-user-auto-expire

gnus-inhibit-user-auto-expire is a variable defined in `gnus-sum.el'.
Its value is

t

Documentation:
*If non-nil, user marking commands will not mark an article as expirable, even if the group has auto-expire turned on.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.