Has ain't gone out of fashion? Teachers have succeeded in stigmatizing the word, and it's also not such a common pet peeve any more. But perhaps the biggest reason you don't hear it as much is because it's no longer used in fiction and movies. Nowadays, it's more common to hear ain't used in certain idioms, like say it ain't so. This is part of a complete episode.
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As usual, I am catching up belatedly with new episodes. But the discussion of "ain't" Â brought to my mind Tom Wolfe's Bonfire of the Vanities where Lawrence Kramer, a Bronx DA, is very conscious of it and he uses on purpose "ain't" when talking with cops and "isn't" when talking with e.g. upper-crust Upper West Side suspects or witnesses.
Same here, tatiana.larina. When I write an article for one trade magazine client, I often use contractions to make the tone more casual. But my editor would not allow the use of ain't and would certainly red-line it (unless it's part of a direct quote). Still, in informal writing, I use ain't without worries. But even then, a red flag goes up in my brain due to years of teachers telling me it ain't a valid word. One English teacher even had a small poster on her bulletin board that said simply: Ain't ain't a word.
By the way, I tried to  use Ngrams to trace the usage of ain't, but for some reason it replaces that with "is not." Not sure if there's a way around that, but if anyone knows how to avoid that kind of filtering by Ngrams, please enlighten.
Heimhenge said
Not sure if there's a way around that, but if anyone knows how to avoid that kind of filtering by Ngrams, please enlighten.
Ngrams offers the raw data via a link at the bottom of the page that I have reproduced here. Perhaps you can find get past their "simple" Ngram filtering by looking at the underlying data.
Ngram is inconsistent with what it's doing- it will modify ain't, aint, don't , but it will leave alone dont.
Once, I put an error message in the computer that said something like 'It ain't so' and the users, all young, protested and made me put in a proper message. It was maybe 1990.
Old fashioned today ? Maybe not. Don't rap artists limit themselves to only aint and dont ?