Tiling Over Plaster Walls Bathroom

Add a diverter and shower line in the wall.
Tiling over plaster walls bathroom. But with cement board. You would have to remove the plaster and replace it not with drywall run from a contractor who says that. You should not tile over wallpaper glossy surfaces lead paint or plywood. The adhesive and tile will block moisture from evaporating.
Bring plumbing to code. If you have a tiling job to do in your bathroom and the walls have been skimmed with plaster you have. The problem otherwise is that grout is porous and any slight crack will cause moisture and mold on any other surface. If plaster is any way compromised you should tear out to studs and start over.
That is true you cannot tile over plaster if that s your plan. You may tile over existing tile painted or unpainted drywall plaster and textured walls. If you re not sure if you have lead paint test kits are available. Cover with cement board tape off.
It used to be very common to put tile up on whatever the wall was and seal the the tile. Tiling on a plaster wall tiling on plaster. However tiling over tile can add quite a bit of thickness so make sure your wall can handle the weight. Fresh plaster whether applied as a patch or to the entire wall must dry completely before tile is applied.
You may not have lathe but compressed board under the plaster and scratch coat. Follow manufacturer instructions to a tee. It is important to check the weight of your tiles as plaster skim has a 20kg per square meter weight ratio. This was back in the 80 s.