Closing large wall gaps

....and don't be surprised if later on there are LEDs that blink/light up even though they are turned off...

Then something on the topic: chip off 20cm of plaster next to the slot on each side. Then nail wire mesh over it. Afterwards apply Rotband in several layers. Let it dry well each time.

...and don’t be surprised if later on there are LEDs somewhere that blink/shine even though they are switched off...

Dimmed lights with the dimmer in/at the switch are also very nice. Flickering light ahoy.

Is this a new build or renovation of an old building?

Due to the radiator, I suspect an old building.

What would also be important to me is if the filling material were so “strong” that it returns some of the structural stability to the masonry, even if only a little, which I cannot imagine with Rotband, for example.

No structural stability will be returned here. Unless you reroute the pipes. How deep was the wall chased into and how thick is the wall? Why was this method chosen at all?

Due to the radiator, I suspect an old building.

Existing building yes, but certainly not an old building with porous bricks. My estimate would be rather (late) seventies.

This is obviously a renovation in an old building. Your concerns and underlying amusement are really super cute, thank you very much for the care! The house, built in 1976, bungalow, i.e. no additional floor, high ceilings, formerly a fiber cement roof, converted to a metal roof, has hardly any structural load. The slit visible in the photo is located in the hallway and is only that wide there; the opposite wall, also load-bearing, also 24cm Poroton brick, is not far from the slitted wall; the next wall behind the slitted wall also comes after 2.5m, runs parallel and forms an exterior wall made of 30cm Poroton brick. The slits themselves consist of 2cm plaster and about 2-3cm brick. Regarding the topic of cable clustering: everything is calculated and approved. Basically, you would be right if I had 2 living rooms, 2 kitchens, 2 bathrooms, several children's rooms, all of which would cause (and support) a certain load, and I would bundle the supply lines of the respective rooms in the hallway. In my case, however, star wiring, there are 4 supply lines per room, so overall the lines are each not nearly as heavily loaded compared to conventional wiring. The loadable total cross-section per room is many times higher. Basically, 5x1.5mm^2 cables were installed; socket circuits with 2.5mm^2. The electrical installation is basically designed to be generous and robust. I would really appreciate it if we could return to the actual topic, closing the slit.

Then chip off 20cm of plaster on each side of the slot. Then nail expanded metal over it. Afterwards apply Rotband in several layers on top. Let it dry well each time.

You mean first fill the slot layer by layer with Rotband; then when I have roughly reached the level of the front of the bricks, put the expanded metal over it and then cover it with, for example, lime-cement plaster/Rotband?