gnus-mark-copied-or-moved-articles-as-expirable

gnus-mark-copied-or-moved-articles-as-expirable is a variable defined in `gnus-sum.el'.
Its value is

nil

Documentation:
If non-nil, mark articles copied or moved to auto-expire group as expirable.
If nil, the expirable marks will be unchanged except that the marks
will be removed when copying or moving articles to a group that has
not turned auto-expire on. If non-nil, articles that have been read
will be marked as expirable when being copied or moved to a group in
which auto-expire is turned on.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.2 of Emacs.