I am looking for the filter that enables me to modify the subject of the "Retrieve Password" notification Email.
If you know which filter to use, please let me know.
I think you're looking for the retrieve_password_title
filter.
For example:
add_filter( 'retrieve_password_title',
function( $title )
{
$title = __( 'WPSE Custom Password Reset' );
return $title;
}
);