It wasn’t quite 40 years ago, but there’s a line in the film The Hunt for Red October (1990) that explicitly describes SOSUS as a submarine “warning net”. Knowing Tom Clancy, I’d guess there are similar references in the book (1984).

If it was meant to be a secret it wasn’t well-kept.

Yes, people knew it's true purpose then. They were wrong about how it was laid out, but people knew about it. It was declassified 30 years ago or so. And much of it was not top secret, just secret. We use entirely different computers and networks for "just" secret vs. unclassified vs. "top" secret.

The exact way they do it now, where they do it, etc. is the secret. The mere existence of a sensitive listening system is not a secret that endangers national security if known; the opposite is true, we want people to know/think they can't move a tennis ball underwater without us hearing it. Knowing enough about it to get past it is such a secret.