I believe we do have something similar -- data centers almost always integrate some form of energy storage (from the venerable battery to fun stuff like flywheels). That kind of stuff is better handled at the building or rack level though I think -- the goal is to maybe backup some workloads and get everything into a safe state.

I remember these being a bit of a nerd cred item in the early 2000s. Googling around I don't see anything too new in this space. I wonder if there was a problem with that approach or if SSDs just filled that niche.

There are products that do this. Gigabyte's I-RAM GC-RAMDISK is one.