They add this black line when an important figure of tech dies. It is the HN equivalent of a black armband: https://en.wikipedia.org/wiki/Black_armband

Today, it's Tony Hoare: https://news.ycombinator.com/item?id=47324054

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It signifies that someone notable in the industry has recently died, in this case Tony Hoare [1]

[1] https://news.ycombinator.com/item?id=47324054