Is an electrical cable allowed under the shower?

In my opinion, it's not totally irrelevant. Just google "VDE 0100-701 Restwanddicke". Cables from other rooms may only be installed in the wall if a remaining wall thickness of 6cm is maintained. Unless they are extra-low voltage or they are grounded and connected via an RCD, which should be the case for an outdoor lamp.

I also believe it is Zone 0 and you must have a distance of 5 or 6 cm. I now have maybe 3 cm.

If the slots are very deep, they should be sealed again with a special insulating mortar. This was recently demanded by an expert on a construction site.

Slots are standard

At least 6 cm of screed will be applied there. Where is the problem?

At least 6 cm of screed goes on there. Where is the problem?

It’s about the shower wall.

There will be 1.5 cm of plaster plus tile adhesive plus tile on it.