I think emails will receive an overhaul before stable to make them more readable. I expect we would add something like "You can unsubscribe from this email in your settings".
An extension could add a link that allows unsubscribing from a given notification type without having to login.
It's also worth noting that (I think) by default all email notifications are opt-in inside Flarum. If a user receives a notification, they enabled it themselves.
