It's not Masonite board???

Nope, just the lath with the wallpaper on top. I've never seen anything like it! The paper is so thick it hides the individual boards underneath, so prior to me scratching the piece off in the picture, we had no idea.

It's called "pasteboard" and it's basically the stuff that thin cardboard boxes are made of. It was often embossed and laquered in fancy houses, or given a faux finish resembling leather.

My guess, if it's over lath, is that they put it up as one of those "good enough" fixes that lasted decades longer than anyone planned.

The glue could be wheat paste or animal glue and might soften with steam or water. After all this time, it would probably take a lot of steam.

Oops, I missed the bit about electric behind it. I'd be wary of water too.

Careful steaming might soften it. Is it boards (butted edge to edge) or lath (skinny with gaps) under there?

You can consider drywall - 1/4 inch - with a skim coat to look like plaster and skip the whole hassle.

Thank you all for your responses! It appears to be boards, there's no gaps between, and one of the adjoining rooms (the dining room) has the exposed boards which we think is charming and adds to the farmhouse feel. The hall is about 4 feet across, 16 feet long, and has 14 foot ceilings. The height makes us nervous about installing drywall. My husband would be doing it himself, I'm disabled and cannot get up on ladders. We were hoping to take off the wallpaper, plaster, do a paneled molding along the bottom quarter of the room, and install a tin ceiling.
We might try the steam technique on one side with no outlets and see how it goes!

shut off the power and have at it. maybe it's Lincrusta, or something similar?