Experience with renovating old building condominiums

So if it was gutted, and according to his statement it was, then the plaster was also renewed. In a gut renovation you strip everything down except the walls.

As for the rest, as I said, ask craftsmen. A competent painter is familiar with exactly these things and assesses them on site. However, I agree that only a few deal with "new" and "bio" and "ecological", but you can find that out too.

We obviously don’t want to knock off all the existing plaster on the walls down to the masonry, but simply make the walls as diffusion-open as possible with minimal work...

Then it’s anyway a question of how diffusion-open the wall structure is if you only apply the final coating, meaning the millimeter of paint. If there is nothing open behind it, because it might be a cement-based plaster, the paint alone won’t help you...

Completely renovated is a good selling point, but not a protected term for what was done. I have already experienced quite a few surprises, as they just plastered with whatever was available. even tile adhesive

If you have doubts, you can clear them up in 5 minutes. Remove some wallpaper in a few spots and tap around a bit. That can be determined quickly.

Year of construction 1905, let's not kid ourselves: originally there were no bathrooms in it. If the overall appearance now looks seamless, then it is at best authentic, but never completely original. And if there is no heritage protection, nothing will have been modernized in a finicky grandfatherly way. If you want "natural" surfaces, then the result will never ever look perfectly styled like it was photoshopped. I stick to my point: if you make one spot too smooth / even, it will only clash with other (undeniably imperfect) spots.

Thanks again for the numerous responses!

Whether during the renovation in the 90s everything was really completely redone up to the masonry, we currently do not know. Possibly not.

Exactly your questions are also the ones we ask ourselves:
How do we determine whether there is lime, gypsum, or cement plaster under the wallpapers?
How do you test that?

And I want to emphasize again:
The goal is not 90° angles and mirror-smooth walls :)

As a side note about the property: built around 1905, but not a tenement with a bathroom halfway up the floor, rather a 3-unit building and even back then each apartment had its own bathroom. I suspect the property was rather built for somewhat better-off civil servants or the like.