You're right. Originally the + sign in an email address was an indicator to the Andrew Message System's delivery agent to process the email in an extensible way. The syntax was +<keyword>+<args>. As an example. you could use
"user+dir-insert+misc" to route the message to the "misc" directory in the user's mailbox structure. An unknown keyword would just get ignored and the mail delivered as usual, giving the behavior as used today.