the following code does not work:

$mform->addElement('static', 'rpd_link', 'Link to RPD', ['class'=>'hidden']);

although the same select works fine:

$mform->addElement('select', 'template_code', 'template', helper::getRequestTemplates(), ['class'=>'hidden']);

This happens because the fourth parameter $attributes is missing from the description of the HTML_QuickForm_static class constructor
although the parent class HTML_QuickForm_element allows accepting $attributes