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In some implementations of markdown underscores italicize the word(s) they surround. On HN this is achieved with asterisks.

I could have sworn I saw it used in email in like 1997 to indicate underscores in plaintext.

Yup. Markdown is directly inspired by this _𝔞𝔫𝔠𝔦𝔢𝔫𝔱_ convention ;)