There used to be a product called ZeusRAM which was a small (like 8GB) SSD with a matching amount of DRAM. In normal use all operations were done on the DRAM, and when power was lost it would use charge held in a super-cap to flush the data out to the NAND.
Oh wow lol, this reminds me of Windows Vista days and Microsoft’s ReadyBoost for using a USB flash drive as extra memory for disk caching because it was hammering peoples disks. Good times.
Haven't seen any purely volatile drives, though.