<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<type name="MagentoEmailModelTemplate">
    <plugin name="mail_subject" type="VENDORMODULEPluginTemplate" sortOrder="100" />
</type>
</config>

namespace VENDORMODULEPlugin;

class Template
{
    public function afterGetSubject(
        MagentoEmailModelTemplate $subject,
        $result
    ) {
        return htmlspecialchars_decode((string)$result, ENT_QUOTES);
    }
}