Gigabyte had a ram disk addin card years ago. Not exactly the same but since it's presented as a storage device you could use it as OS swap space.

https://en.wikipedia.org/wiki/I-RAM

Adding the RAM to the system this way isn’t exactly like expanding the main system RAM. The RAM is connected over a PCIe type link so it doesn’t behave like the primary RAM.

It’s better for server farms where engineers can customize and tune for an architecture like this.

There have been some cards that use RAM as a storage device. They were never popular because having to set it up as a disk had very limited use cases.

The sensibility is a bit different since consumer systems don't really have much bandwidth back to the CPU. Given the current resale prices of DDR4, might as well just sell it and get some NVMe drives.