It certainly is a frequently asked question. But spelling everything out is against the spirit of the site.
On HN it's good for readers to have to work a little, and the audience is smart enough to figure things out.
It's equivalent to the flag at half mast:
> When someone important to the community dies, a thin black bar is added to the top of HN as a mark of respect
Welcome to HN!
I love this question every time I see it.
Paul Allen, co-founder of Microsoft, has died.
It's for the late Paul Allen RIP.
Should've known, sorry all.
I'm relatively new to HN, and as such I hadn't encountered the black line before. Thanks for asking the question so that noobs like me can find out the answer.