Almost looks like a hardboard (Masonite) type product. No idea why they would have used it but you could do either. Skim coat if it is not smooth or remove it and put up drywall if you want.

1990? It’s Sheetrock that someone painted high gloss gold. The next coat didn’t seem to bond well so I would abraid off what you can and prime well before too coating.

Looks like you havent removed all the wallpaper backing either. Try to wet it down and see if you can remove all that, down to the gold painted surface.