Space under the stairs not buildable

If it absolutely must be drywall - perhaps another material is more suitable and Company 2 has a good idea up its sleeve.

hm, we only needed a lower wall... we also have the upper half with risers, below that is the wardrobe. The rest is brickwork.
We are very satisfied! That makes it practical!
For example, we can see on the stairs when the light underneath is on, etc.
Many picky armchair critics here would rate that as insufficient. Or deficient. No idea. It is also a special request for the general contractor, which was carried out by the drywall installer. Otherwise, no one touched it. We could probably spruce it up, but it's good for us.
If you are interested, I can take photos.

If you read some threads where builders try to find defects in order to withhold money, I can understand if a provider does not agree to such a pragmatic and functional solution.

hm, we only needed a lower wall... the upper half also has risers, below that is the wardrobe. The rest is masonry. We are very satisfied! That makes it practical! For example, we can see on the stairs when the light underneath is on, etc. Many picky armchair critics here would rate that as insufficient. Or poor. No idea. It is also a special request for the general contractor that was carried out by the drywaller. Otherwise, no one else worked on it. We could probably dress it up, but for us it's fine. If you're interested, I can take photos.

I’d be happy if you could take a photo. :) Here is what we are planning:

When you read some threads where builders try to find defects in order to withhold money, I can understand if a provider does not agree to such a pragmatic and functional solution.

I would not count us among such builders ;)

hm, you just need to attach the panels inside and seal the stringers of the staircase from below or the piece at the top with acrylic. ...