If you look at Google’s N-Gram viewer, which tracks the usage of words and phrases within the Google Books corpus, we can see that the phrase “So it goes” was popular long before Slaughterhouse-Five was published in 1969. If we remove smoothing and zoom in, we do see a pick up in the printed use of the phrase in 1969, presumably tied to the publication of the book (i.e. the phrase is used in reviews and references), but only as its own sentence with a capital S.

The phase seems to have been slightly more popular, at least in its printed form (it’s harder to argue about the popularity of spoke phrases), in the period of roughly 1920 to 1945, roughly Vonnegut’s youth (he was born in 1922) and also the setting of the core part of the story (1944-5). The book does seem to have led to a slowing of the phrase’s decline, and even a temporary resurgence, but it’s definitely a phrase with a long history of common English usage. All of the cultural usas of the phrase that I could find (a British musical program, a Perry Cuomo album, a Taylor Swift song, a Billy Joel prose poem) post-date Vonnegut’s novel.

Shortly after Vonnegut’s death, the Guardian argued that the words “could most fittingly serve as his epitaph” as they imply the themes essential for understanding his work: “fatalism, stoicism and the acceptance that no use will come of shrinking away when the worst has happened.” So it goes.