So I noticed my forum boardindex is marked robot noindex. That is xxx.xxx/forum or xxx.xxx/forum/index.php.
I think it's a bug, but I'm not sure so I'm posting it here. Move to bug if it is.
The no index is coming from BoardIndexController.php
// Do not let search engines index anything if there is a random thing in $_GET.
if (!empty($this->_req->query))
$context['robot_no_index'] = true;
req->query always has start=0 in it. This is set in Request.php
// There should be a $_REQUEST['start'], some at least. If you need to default to other than 0, use $_GET['start'].
if (empty($_REQUEST['start']) || $_REQUEST['start'] < 0 || (int) $_REQUEST['start'] > 2147473647)
$_REQUEST['start'] = 0;
However even commenting out the above in Request.php still causes the robot_no_index to be triggered in BoardIndexController.php even though printing $this->_req->query shows an empty array. However, replacing the check to match the comment and checking if $_GET is empty then the noindex is not triggered if there are no parameters.