There was a version of optane (optane DC?) that plugged into DIMM slots and cost less than RAM, with possible higher capacities. That would be a useful tradeoff had it kept up the advantage.

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.

Intel had multiple false starts on getting their platform ready for these devices to be used in a sensible manner. I have one of these false starts in a closet somewhere, and I did some kernel dev work for it. It was an incredible mess.

(I didn't actually have an Intel DIMM module -- I had a different vendor's. But fundamentally there needed to be a clean way to get data from software into the durable memory domain, and there wasn't.)