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"
He is definitely not forgotten in Heidelberg, where he lived and worked until 1948.
He's not literally forgotten but few read him compared to Heidegger and Sartre. Some of his writings aren't translated into English or are very rare.
fwiw I consider myself somewhat of an existentialist and I've never heard of him. Then again, I didn't manage reading Sisyphus, too boring and.. french.
It's a bit of a slow burn, but the final section (the bit actually and Sisyphus) is beautiful. It's worth pushing though the earlier chapters to get to it.