Yes, it's stainless. It was very popular as high-end stainless. It was used a lot by Benchmade and more popular with kitchen knives in the past.
You may find more information looking up 154CM (non-powdered version). ATS-34 (Hitachi version). Also known as RWL-34 (Damasteel).
I have zero evidence but was led to believe it was 19C27. It was my understanding the extra hard and extra thick/thin versions were the knives from batches that were outside of the normal variances. A beneficial error.
In the end it didn't matter what stainless it was to me. It sharpened easy...
The YS is the favorite knife of the owner of Ai & Om knives. He's a former chef turned knife retailer if that helps. His go to was an HD for years until the YS was released. He convinced me to buy it but I didn't have the coin. It's a stiff stout blade compared to the much thinner Konosukes.