I found another one! And once again in a novel by Marilyn French (1929-2009), this time My Summer with George (1996). The novel is situated in 1991 (p. 236), and it is about the dream of an affair in late middle age with a man called George, who declares himself a hater of women, poor-self-deluded Hermione!
During a conversation that appears to be getting nowhere, George …
told me about a quarrel at the newspaper among a group of editors, about a split infinitive. He wondered how I felt about split infinitives; he seemed seriously interested. So I, too, treated it seriously, explaining that being of the old school, of course I disapprove of split infinitives; they invariably hit my ear as crude and déclassé. I offered my old-fashioned opinion for what it was worth, and he launched into a series of grammatical questions that plagued the editorial desk at Newsday. As he paid the check, I remarked that it was impressive that the editors would argue about such high-flown subjects. I thought grammar was no longer of interest to anyone except a few linguists talking to each other by E-mail, I said (p. 205).
And so on. “Crude and déclassé”, in the early 1990s, fit for a topic in this empty conversation, followed by the comment that grammar was not believed to be of any interest except to the occasional linguist. This reads like a critical comment on the absence grammar as a subject in the school curriculum at the time. George raised the topic because he had sat next to a former “grammar teacher” on the plane to New York, who got “all agitated about the grammar in a column in the Louisville Herald – my paper! I was insulted!” (p. 206).