I wouldn't have thought that such a small a bit of modern plaster / filler is going to significantly impair the permeability of the wall as a whole (particularly as most modern paints are going to be doing that anyway).
There are some bits where the plaster has blown and pulled away when we’ve removed wallpaper - would you still think that bonding and easifil be the way to go?
We’re going to try and use clay paint where budget permits as well.
I wouldn't have thought that such a small a bit of modern plaster / filler is going to significantly impair the permeability of the wall as a whole (particularly as most modern paints are going to be doing that anyway).