Slaughterhouse Ubik
Slaughterhouse-Five by Kurt Vonnegut, which was published in 1969, and which is, in part, a novel about time travel, "continually employs the refrain 'So it goes.' when death, dying, and mortality occur, as a narrative transition to another subject, as a memento mori, as comic relief, and to explain the unexplained. It appears 106 times."
Ubik by Philip K. Dick, also published in 1969, and also, in a way, about time travel, contains the following paragraphs:
from p152: "Joe said, 'It's fairly easy to establish that. I took a look at a newspaper at my conapt back in New York. September 12th. So today is September 13th, 1939. The French think they've breached the Siegfried Line." ¶ 'Which, in itself,' Jon Ild said, 'is a million laughs.' ¶ 'I hoped,' Joe said, 'that you as a group were experiencing a later reality. Well, so it goes.'"
from p164: "He wondered if he had made the right choice; now, too late, he considered the alternative, what it might have meant. A return--possibly--to his own time. Out of this devolved world of constantly declining time-binding capacity--out, perhaps, forever. Well, he thought, so it goes."