I think I know which you're talking about, but Koko herself wasn't really a hoax; that silly, politicized "speech" was unconscionable, though.

There were also Washoe (and co.), as well as Kanzi (still around; he started with plastic symbols, and more lately started using computer boards starting in the 90s or so - he's the one who was also taught stone knapping).

I guess most of the information got buried, but the best account was done by Eugene Linden (Apes, Men and Language and Silent Partners), both hard books to get a hold of now outside of libraries.

There were MANY more projects that those but those three were the rock stars; I remember Linden describing one colony whose purpose was to teach the adults ASL, and then see if they'd teach their offspring (with no input from the researchers, the apes would be on their own). Turns out, they certainly did teach signs to their kids, but funding was cut in the middle of the experiment, and thanks to donations, they were sent back to Africa. And set loose. Where no one has been bothering to watch them since, I don't think. Yep, it's where Chrichton got the idea for Congo from (he thought gorillas would be more impressive for a story).

It's a very old rabbit hole for me, but I still see stuff around; the studies on wild apes are pretty interesting, though. Did you know savannah apes use spears?