wp-includes/PHPMailer/PHPMailer.php:4253Check if this message has an alternative body set.
bool public function alternativeExists() { return !empty($this->AltBody); }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.