That's the persistent memory modules; they were roughly half the price per GB compared to DDR4 from my quick research, but substantially higher memory latency, which would significantly affect usage as a KV cache, which would probably be their primary use case since SSDs beat it in raw data throughput. Who knows how it would have developed over the years since it was discontinued, but my gut reaction to the numbers said you'd rather just invest in more RAM based on the numbers I saw.