I got it.
/engine/lib/users.php
set_user_notification_setting($user->getGUID(), 'email', false);
set_user_notification_setting($user->getGUID(), 'site', true);
;)
I've been looking for the same thing. If someone is interested, it's now here:
/engine/classes/Elgg/Database/UserTable.php
line 434:
// Turn on email notifications by default
set_user_notification_setting($user->getGUID(), 'email', true);
Although it might not be the best idea to change the core code, I'm still looking into solving this through a plugin.
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