The filter can be used if it’s need to change the subject while sending the subscription email.
Changelog
Since 7.6.1 version
Parameters
1. $subject – the subject of the subscription email
2. $comment – the newly added comment
3. $emailData – the data of the subscription email
Usage
add_filter('wpdiscuz_email_subject', function($subject, $comment, $emailData){
return 'My Custom Subject';
});