Computers in the 50's and 60's used core memory, which was persistent. I was surprised to learn that it was common practice to "pause" computations over the weekend by writing a few bytes to the "boot sector" so that the computation could be restarted later.
I have some core from the seventies on a shelf. I often wonder if the residual magnetism is readable.
Depends on the temperature.
And vibration and presence/absence of strong magnetic fields over the time since it was written.