wp-includes/PHPMailer/PHPMailer.php:965Sets message type to HTML or plain.
$isHtmlbooloptionaltrue public function isHTML($isHtml = true) { if ($isHtml) { $this->ContentType = static::CONTENT_TYPE_TEXT_HTML; } else { $this->ContentType = static::CONTENT_TYPE_PLAINTEXT; } }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.