SubjectRewrite in 3.0.x
The syntax for enabling Subject rewriting has changed in 3.0.x, it is now a more generic rewrite_header option.
This is documented in the [http://spamassassin.apache.org/full/3.0.x/dist/UPGRADE UPGRADE file]:
The "rewrite_subject" and "subject_tag" configuration options were deprecated and are now removed. Instead, using "rewrite_header Subject [your desired setting]". e.g.
rewrite_subject 1
subject_tag ****SPAM(SCORE)****
becomes
rewrite_header Subject ****SPAM(SCORE)****