ShowInSearch field only affects built in search functions, so no change required!
On the other hand if you do want the ‘show in search’ to affect search engines, just put this in the head section of Page.ss:
<% if $ShowInSearch != 1 %><meta name="robots" content="noindex"><% end_if %>
You can also optionally tweak the description with this in Page.php:
public function getSettingsFields()
$fields = parent::getSettingsFields();
$fields->replaceField('ShowInSearch', CheckboxField::create('ShowInSearch', 'Visible to Search Engines')); // Behaviour handled in Page.ss
(sorry, I know you didn’t ask for that but I originally misread the question and already typed this out )