Seamless bathroom. Sealing in the bathtub area

Let's assume the wall gets wet daily from showering. Would the plaster and the paint then also be unaffected? Provided it is always ventilated.

Then my answer is: poorly planned bathroom if there is no shower. And poorly planned wall construction if water is splashed there daily in significant amounts (is the wall behind the plaster waterproofed?).

And then I would also set aside both my suggestions and yours and put tiles on the wall. Alternatively, a microcement coating (have it done). But that should have been considered beforehand.

To answer the question specifically: with this usage, you will get water stains on the other side of the wall if there is no waterproofing behind the plaster. And no, the plaster does not tolerate exposure like in a shower—this is why showers are tiled 99.8% of the time and the wall is properly sealed accordingly.