Hacker News Reader

Click "Lists" In the footer of the site; it's no secret.

It's incomplete though - https://news.ycombinator.com/classic is not listed, for example.

Is there a reason for keeping these hidden?

I've also wondered about downvoting... I never see a button for that, but it seems like some people are still able to. Is it just the 'flag'?

No, you need a minimum amount of karma for the downvote button to appear. I think its at ~500 now.