Volatile memory - like DDR4 RAM - requires constant power to hold data, the data is corrupted or lost once power is removed. There are forensic methods of recovering data from unpowered RAM, they usually involve quick action and extreme subzero temperatures and laser-etching the packaging down to expose the silicon and optically extract/reconstruct the stored information - the sort of stuff you'd expect some MIB/NSA/CIA agency might be able to pull off. There are no software methods which can recover the contents of RAM after power is shut off. In fact, most computers electrically "erase" the RAM contents during POST every time they reboot.

Nonvolatile memory - like SSD storage and NVDIMMs - only requires power when (re)writing data, they can hold data indefinitely without power.

If RAMCache operates entirely within volatile RAM then it can't retain any cached information after power loss. And there's no point in running RAMCache with nonvolatile memory types because it would offer no performance benefit.

Some SSD brands offer caching software - like Samsung's RAPID Magician - which can cache SSD data in (volatile) RAM. Some of these can be configured to permit Read Caching and/or Write Caching independently - enabling both offers best performance gains but enabling Write Caching risks data loss if the cache cannot be written before sudden power loss.

Some physical RAM disk devices still exist which actually run a virtual disk in volatile RAM DIMMs. Most of these feature some sort of battery or NVflash backup system to prevent data loss during sudden power loss events, but these backups can be deliberately disabled if "destroying" data (for paranoia/security reasons) is important.