Reddit reviews Zinsser 2976 Paper Tiger Free-Floating Self-Aligning Triple Head Wallpaper Remover Tool, 1 in L X 3 in W, Steel Teeth

You can get a spiky tool that will puncture the wall paper and make it possible to either spread a wallpaper release agent or steam the wallpaper off.

I don't know anything about any of those products, but figured they would give you a place to start more research.

EDIT: you will probably need to skim coat some areas of the walls and retexture.

https://www.amazon.com/Zinsser-2976-PaperTiger-Scoring-Wallpaper/dp/B0002YWWJG

Basically it's like a hockey puck that you hold in your hand, but there's a little caster inside with gears for wheels. The gear teeth are sharpened and only long enough to puncture wallpaper without harming the paper on the plasterboard. You hold the "puck" in your hand and rub it all over the wallpaper. All those little holes make it easier for steam or glue remover to penetrate the paper. It's a must for very thick wallpaper or the really glossy, waxy, plastic-y kind.

I agree with this. Perf the paper, then steam. Some of the older paper is quite thick and needs perforation before steaming, don't go too deep tho!~

Perforator

You can kind of do the same thing with a sharp blade, but be very careful not to score too deep, if anything a scratch will do.

One more tip... spray the wallpaper after scoring with a light mist of water, then apply your steam. Move your steamer nice and slow. Tie Chi, not karate.