[jeffbr13]

Yep, as the Google N-Gram view in the article shows, usage of "theater" overtook "theater" in the late 1970s, but the two had quite similar usage rates from the 1940s onwards.

Style guides generally update to represent the editorial team's preferences, to reflect changes in the wider world (e.g. see the comment in this thread about "Constaninople"→"Istanbul", or to make a point (see the New Yorker's continued use of diaresis in coöperation etc.[1])

[1]: https://en.wikipedia.org/wiki/Diaeresis_%28diacritic%29#Engl...