/etc/mail/spamassassin/custom.cf will remain after restart.
Read more about creating your own spamassassin rules.
https://cwiki.apache.org/confluence/display/SPAMASSASSIN/writingrules
https://gist.github.com/jult/9bfdc4d07b44be01a02cc2aaf25b7c39
https://github.com/kawaiipantsu/spamassassin-rules