wp-includes/PHPMailer/PHPMailer.php:4267Clear all custom headers.
public function clearCustomHeaders() { $this->CustomHeader = []; }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/PHPMailer/PHPMailer.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.