wp-includes/PHPMailer/PHPMailer.php:4325Check if an error occurred.
bool public function isError() { return $this->error_count > 0; }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.