Someone who had a flag applied to them was flagged.
Noun to verb conjugations are unofficial English. But we love verbing our words. Google has been verbed into Googling. And when someone is the target of Google we call that getting Googled.
It's again, unofficial and unwritten and unstudied American English. So don't sweat it. It's how our slang evolves. Apologies for the lack of consistency in our language!
Hm? Where'd you get that impression?
It's certainly unofficial -- English has no official governing body, so essentially _all_ English is unofficial. However, zero-derived denominal verbs are quite common in even formal written English [0], are well studied [1], and aren't at all limited to American English [2].
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[0] As seen in this very thread.
[1] It took me almost no time to find a random academic paper [1a] and two Wikipedia articles [1b][1c].
[1a] https://dingo.sbs.arizona.edu/~hharley/PDFs/HarleyDenominalV...
[1b] https://en.wikipedia.org/wiki/Denominal_verb
[1c] https://en.wikipedia.org/wiki/Conversion_(word_formation)
[2] https://www.thetimes.com/uk/politics/article/the-pedant-noun...
I do feel there's a pattern of me reading some interesting tech news, then thinking "wait, why didn't I see this discussed on HN?", to searching for it and finding a buried/flagged HN discussion due to it being somewhat tied to politics (what isn't?)