There was a product like this, the Gigabyte i-RAM. It used sticks of DDR-200 to DDR-400 and put them onto SATA-I as just a regular drive. You could use it as a ReadyBoost-like drive cache with the use of software, It came with a battery backup.
It was most fun back in the days when you could install Win9x to it as it had 1/10th the latency of even the fastest SSDs. Later on I used it to put a pagefile on for 32-bit systems which could only access 4GB of actual RAM, however when system RAM became cheap enough I instead used RAMdrive software to put the pagedrive on otherwise inaccesible RAM above 4GB. I never was able to use it for ReadyBoost itself but heard of some workarounds to allow that
BTW just about every HDD in the world does already have a RAM cache, usually something small like 16MB. The i-RAM allowed 4GB, and RAMdrives are only limited by how much system RAM you'd like to dedicate for a drive cache. Obviously it makes no sense to put a pagefile onto a RAMdrive of a 64-bit system, but some poorly written programs do benefit from a scratch disk there.
This is a good question because all of the usual technologies you can think of for this used memory with a limited write cycle life, such as Intel Turbo Memory (SLC), Condusiv ExpressCache/Sandisk ReadyCache (which used MLC!) and Intel Optane (3D XPoint). There have also been various hybrid drive arrangements to combine a HDD with some flash such as Apple Fusion Drive or Intel SRT which could presumably be used with an i-RAM-like device, and of course Seagate had their SSHDs which integrated a large SLC cache into a HDD.