I had a pre-1920’s house that was papered. I didn’t have plaster beneath the paper, I had concrete. If the walls were originally papered, then you might have lathe, portland cemet and then paper on that. If there’s plaster, it’ll be a finish layer on top of the portland.
If you can peel a corner up, you can tell by the color, the portland is sidewalk gray and looks very much like cememt.
Our wallpaper was literally paper, not vinyl on paper. I rented a steam machine from the rental store, got a wide bladed scraper from the hardware store and just kept steaming and peeling. The portland is nearly impervious to the water so it does very little damage to the wall. (Careful, the scraper gets sharp from being “honed” on the rough wall).
Unfortunately, when the walls were clean & dry (let’m dry for a long time) the bare walls look like a sidewalk. Painting them would’ve just looked like a painted sidewalk, not smooth.
This left me with a couple of choices, paint them and call it “textured” (not very decorous, tho), hang more paper, or plaster them.
Plastering is nearly a lost art, I couldn’t afford to get it done & I couldn’t do it.
We hung more paper. I probably could’ve hung the paper on top of the other & saved the scraping process. But - since I did scrape - I had to hang the paper on the portland.
The wall-paper store said we had to seal the portland with an acrylic sealer so we did. We then hung the paper and it was still sufficiently porous that it sucked the moisture out of the glue. The paper shrank (yes, we “booked” it first) and the edges pulled apart leaving us with 1/8th-inch gaps between every sheet.
When we did the next room over, I sealed the portland under two coats of latex paint, then papered. It worked much better.
Maybe two or three coats of sealer would’ve helped the first room. One certainly wasn’t enough.
One option for covering portland walls, by my recollection, is to cover them with wallboard. The wallboard makers make a 1/4th-inch stuff that’s good for this. It’s made for remodeling and will conceal the cracks & stuff that occur in old houses’ walls from settling. You can paint it afterward & it looks like regular plaster or wallboard walls.
If you want to do this (paint the walls), then you might just take a sledge to the wall & break the plaster down and hang standard 5/8th’s wallboard in its place. Mud, sand & paint and you’ve got new walls.
I’ll bet a house as old as yours has no insulation in the walls and if you expose the wall studs, you can insert insulation in there.
HTH - B