David Gale wrote:

Last night, as I was doing some electrical work, I decided I needed to open
a wall up. So I pulled out my utility knife and started cutting out a
section; to my surprise, it wasn't drywall (or plaster), but rather some
sort of cardboard-like substance. It's about as thick as normal drywall,
held up by drywall nails. What is this, and how long ago were people using
it? Are there any potential health hazards I should be aware of when
working on/around it?

Probably homasote, or some other pressed celulose board.
If it's making a lot of dust, wear a mask.