You would have to remove the plaster and replace it not with drywall run from a contractor who says that.
Tile over plaster walls.
I have repaired tile jobs that were over 20 years old that the tile was applied directly to regular sheetrock and plaster walls.
Then rinse the wall with a water dampened.
Bring plumbing to code.
This was back in the 80 s.
Layer newspaper on the floor across the bottom of the tiled wall.
It used to be very common to put tile up on whatever the wall was and seal the the tile.
Cover with cement board tape off.
That is true you cannot tile over plaster if that s your plan.
Add a diverter and shower line in the wall.
Also now it s been plastered you will be restricting yourself to a tile with a max weight of about 16 kg m2.
Can you lay tile on plaster walls.
Follow manufacturer instructions to a tee.
The problem otherwise is that grout is porous and any slight crack will cause moisture and mold on any other surface.
If you re not sure if you have lead paint test kits are available.
Both painted and bare plaster allows for the installation of tile.
Don t use paint glue pva wall paper paste only use the primer recommended by the adhesive manufacturer of your chosen tile adhesive.
Scrub the wall tiles with diluted degreasing cleaner and a nylon scrub brush.
Loose paint must be scraped.
If plaster is any way compromised you should tear out to studs and start over.
You may tile over existing tile painted or unpainted drywall plaster and textured walls.
Mix degreasing cleaner with water in a bucket using a dilution recommended by the cleaner manufacturer.
Testing the wall for durability.
You should not tile over wallpaper glossy surfaces lead paint or plywood.
Not all plaster wall surfaces will support the tile properly.
Or flatten walls with a latex modified skim coat product.
Mold does not grow on cement.