The gerrit change was abandoned, so if this is still relevant we'll need it filed on Phabricator...
Yeah, this is still absolute nonsense and every time I access my settings I start to loose faith again. In other words: Thanks for notifying. I will file a task.
Ok. I suspect you meant MediaWiki:Gender-unknown/en though. I think the parentheses are there to convey the fact that it's a non-choice, the default choice, or not a direct answer to the question above the radio buttons. Then I don't know how consistent this is with the other preferences etc.