Our 1905 row house was "renovated" in the 70's, meaning they threw away the balusters and handrails, replacing them with drywall. They covered the stair tread with particle board and industrial carpet. pulled the drywall down on the third floor and the stairway from the 3rd to the second floor, then removed the carpet and particle board.
I found that the particle board had been shimmed up quite a bit to make the stairs level. With it removed, I have a slant of almost 1-1/2" across a 31" stair tread! It has three stringers and I can see the top end of the stringer on the side that is low. It seems tight against the floor joist. I haven't opened up the bottom end yet.
Complicating matters, the stringers were apparently not cut quite correctly 100 years ago. On the end that is low the original treads are shimmed up 1/2" to 3/4". If I pull the treads and these shims I may get a 2" drop! I hate to replace the stringers. They are full-2" width old growth, rough-sawn wood. My plan is to try to jack up the lower end of the stringer so as to reduce the slant, then just shim up the new treads to make them level.
The other option is to rip out the two stringers that are low and replace them.