Are retaining walls on slopes prohibited according to the development plan?

One doesn't know the property here either. 10% is quite steep, but what about the building envelope, in which direction could the terrace go? ...because you can place the entrance to the basement, etc.... So: you really have to move away from the standard assumptions and explore options. Many hillside properties become buildable this way, often with a high added value. However, it also costs more.

So over the entire property, the average slope would be between 6 and 7%. If the house (one meter above street level) stands and the terrace is behind it (south of the house, unfortunately uphill), the steeper slope of just under 10% remains for the garden. Unfortunately, the house itself may only stand up to one meter above street level.

For 10%, you don't necessarily need a wall. A small sloping strip at 50-60 degrees - depending on the location - is also sufficient. Behind it, the fence, some vegetation on it, and done.

The planners do not want high walls and brutal filling. I do not find 1.2 m much. That is what I have from the street to the garage

So over the entire property, there is an average slope of between 6 and 7%.
If the house (one meter above street level) stands and behind it the terrace (south of the house, unfortunately uphill) remains, the steeper slope of just under 10% is left for the garden. Unfortunately, the house itself may only be a maximum of one meter above street level.

unfortunately, this does not answer the open questions, but is only a subjective point of view from someone who is frustrated with the development plan

but otherwise I can’t find any further information on it anyway

I do. It really couldn’t be formulated any more clearly:
B.4.2 Excavations, Embankments
Embankments of max. 0.5 m and excavations of up to 0.5 m to level the ground floor to the natural terrain are permitted.
The natural terrain must be strictly maintained at the respective property boundaries.

Unfortunately, that’s because every time I read the statute and try to interpret it for myself, I interpret something different…

That’s why it was a good idea to turn to the forum. But you also have to give those experienced in interpretation a chance.

Unfortunately, the house itself may only be up to one meter above street level.

Have you already picked out a plot? Then this interpretation may be correct, but what is stipulated is:
OK finished floor ground floor = max. + 0.15 m above the highest point of the natural terrain within the planned building footprint.
Although the planning status still seems to be far from the public display of the development plan. So you still have opportunities to influence the statute.

1.20m over 13.00m length is not much. Slope it off, as suggests, and you can save a lot of money on walls.