Use this code example to turn it off. You see the change on forms that don't show the clickable list of field errors. On a form with the list, field errors already have no alert role.

add_filter( 'frm_include_alert_role_on_field_errors', '__return_false' );

To check it, submit the form with an empty required field and inspect the error message under the field. Only that message changes. The error summary at the top of the form keeps its alert role either way.

Use this code example to add the alert role back to field errors on forms that show the clickable list of field errors.

add_filter( 'frm_include_alert_role_on_field_errors', '__return_true' );

Use this code example to turn it off on one form only. Replace 5 with your form ID. The code checks $form first because it's null when Formidable sets the default for the whole page.

add_filter( 'frm_include_alert_role_on_field_errors', 'remove_alert_role_one_form', 10, 2 );
function remove_alert_role_one_form( $include_alert_role, $form ) {
    if ( $form && $form->id == 5 ) { // change 5 to your form ID
        return false;
    }
    return $include_alert_role;
}