This is no idle issue for Mary. She is not only a linguist; by some quirk of fate, her own last name ends in s. Norris’ or Norris’s?
In English, the situation with generally accepted standards is complicated. Thus, every major publishing company has to develop its own regulations or follow any of the already existing style guides.
At The New Yorker, this burden lies with Mary Norris. She works there since as far back as 1978. Due to her thoroughness and diligence, she has been dignified with the title of the Comma Queen. There is even a separate article about her on Wikipedia.
Mary Norris has her own playlist on the YouTube channel of The New Yorker with an obvious name—Comma Queen. It contains 2–3-minute videos where she explains the most amusing aspects of English, such their as a singular pronoun, Oxford commas, who/whom, etc.