There isn't an option in Email Preferences to mute project subscriber changes. There is one for project membership and watcher changes (once again a matter of making sure subscribers and watchers are kept different; T6113) but this is different.
Kind of funny: in Wikimedia users are discovering that they can unsubscribe from a project while keeping its membership (and receive less notifications by doing so), and this is causing... more notifications for those who stay as subscribers, who get more annoyed and unsubscribe, etc.. :)
Original report: https://phabricator.wikimedia.org/T102180