message-subject-trailing-was-query

message-subject-trailing-was-query is a variable defined in `message.el'.
Its value is

t

Documentation:
*What to do with trailing "(was: )" in subject lines.
If nil, leave the subject unchanged. If it is the symbol `ask', query
the user what do do. In this case, the subject is matched against
`message-subject-trailing-was-ask-regexp'. If
`message-subject-trailing-was-query' is t, always strip the trailing
old subject. In this case, `message-subject-trailing-was-regexp' is
used.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.