Since it is painted over already, it will not matter much to paint over it again.

I have an old house from 1780. We bought it last year. Every room is wallpapered. We removed the paper in the kitchen and one bathroom. These rooms had sort of a semi-gloss paint under wallpaper and it wasn't that hard to remove the paper. Now I started in the upstairs hallway and boy that paper is stuck but good. Raw plaster (horsehair) underneath. So I call in the pros who had given me a $300 estimate to remove the paper in both of the hallways and up the stairs. Well after working on just one long wall, they say that it will cost about $3000 since the paper is so well-adhered. So I say stop stripping the paper and what are my options. They suggest sealing the paper and painting over it. I said fine, I can do that. So I tested some primer on it and nary a bubble appeared. So my solution is to just paint over it. Not my preference, but definitely easier on my wallet!

Does painting old wallpaper make it much harder to remove later?

YES