Karl Jasper's role in developing existentialism is noted well in "At the Existentialist Café" by Sarah Bakewell.
He's also known as the mentor of Hannah Arendt, who later wrote "Origins of Totalitarianism", "The Human Condition" (Alan Kay's favorite book) and "Eichmann in Jerusalem" (served as basis of her 2012 biopic)
He isn't really forgotten in the academic circles who deal with the history and philosophy of existentialism. Jaspers is very well remembered along with Kierkegaard, Sartre, Camus and others. But yes, at the 'general public' level, he isn't really forgotten.
Is he really that forgotten? Camus talks about him in "Myth of Sisyphus" and while I haven't read his work personally I definitely recognize his name from the "existentialist pantheon"