I have repaired tile jobs that were over 20 years old that the tile was applied directly to regular sheetrock and plaster walls.
Tiling over plaster walls bathroom.
Follow manufacturer instructions to a tee.
If you re not sure if you have lead paint test kits are available.
You may not have lathe but compressed board under the plaster and scratch coat.
That is true you cannot tile over plaster if that s your plan.
Bring plumbing to code.
But with cement board.
This was back in the 80 s.
It is important to check the weight of your tiles as plaster skim has a 20kg per square meter weight ratio.
If plaster is any way compromised you should tear out to studs and start over.
If so you will need to paint the wall with pva glue tip some ova into a bucket and thin it down a little with water so you have a creamyish texture then paint onto the area you wish to tile and allow to dry it should be dry enough to start tiling after half hour so start tiling fro the bottom and work up true ova seals the plaster and will give you a surface for the adhesive to adhere to hope this helps.
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.
Cover with cement board tape off.
You should not tile over wallpaper glossy surfaces lead paint or plywood.
It used to be very common to put tile up on whatever the wall was and seal the the tile.
However tiling over tile can add quite a bit of thickness so make sure your wall can handle the weight.
You would have to remove the plaster and replace it not with drywall run from a contractor who says that.
You may tile over existing tile painted or unpainted drywall plaster and textured walls.
Fresh plaster whether applied as a patch or to the entire wall must dry completely before tile is applied.
The adhesive and tile will block moisture from evaporating.