Default robot policies ($wgNamespaceRobotPolicies) (T122119) as well as __INDEX__ & __NOINDEX__ (T122252) are not applied in Flow boards.

Difference between Flow & other pages is that Flow content is in multiple pages: a topic is in the board, and in a Topic:xyz page.
I'm not aware of any possibility for partial search engine directives, where parts of a page are indexable and other are not, so we may have to support it only on a board level and let it trickle down to Topic:xyz pages?

Excerpt from Article::view, although we'll likely have to extract __INDEX__ & __NOINDEX__ from header descriptions ourselves still:

# Check for any __NOINDEX__ tags on the page using $pOutput
$policy = $this->getRobotPolicy( 'view', $pOutput );
$outputPage->setIndexPolicy( $policy['index'] );
$outputPage->setFollowPolicy( $policy['follow'] );