The thing with using NAND as a high-traffic swap cache is the idea of one write, many reads.
Which is why Readyboost cache was a thing, which is why the Corsair Accelerator existed and why the Sandisk Radycache was rather popular for awhile.
Add onto the mechanical drives with a chunk of NAND like the Seagate Firecuda SSHD.... And it's actually a common idea.
Optane is NAND which is optimized for it's task, rather than using commodity chips.
I'm surprised that you deleted the comment that I responded to, which was treating SSD "like Optane"...
Opatane is NOT NAND; It's 3D Xpoint.
Not only is Optane faster, when used as a HDD cache, it contains vastly different caching methods compared to SSHDs to make use of the magnitudes greater TBW compared to NAND. That is why Optane can achieve speeds much greater than SSHDs. Caching via SSD is NOT done "like Optane".
Please do your research before arguing your point.