Should a hanging plot be filled for a ground slab?

A fill embankment does indeed hold on its own if it is professionally applied and compacted in layers. You can get plenty of a soil suitable for compaction because many civil engineers don’t know what to do with it, and even more property owners would love to save expensive landfill fees with it.
We added and compacted about 700 m³ on our property. That saved about 90k for the basement.

But you do know that you can also use the basement for those 90k, right? :D
And that the heating etc. is cleverly hidden, not in the ground floor? I would never ever fill a hillside property. Really a shame :(

Have you considered whether you could partially build the lower floor into the slope instead of the basement? The rooms without or with little natural light can be used for technology / storage or similar.

You need 8-9 m for the runoff. Does your property allow for that?
Then you still have a 30-35% slope. How do you want to maintain that?
You want an age-appropriate property.

Even if compacted every 30 cm (hand-operated equipment) up to 50 cm (self-propelled, e.g. vibrating rollers), I would not build on the property immediately, but let the slope grow in and allow the compaction to settle.

You need 8-9 m to run out. Does your property allow that? Then you still have a 30-35% slope. How do you want to maintain that? You want an age-appropriate property Even if it was compacted every 30 cm (hand-held equipment) up to 50 cm (self-propelled e.g. vibrating rollers), I would not build on the property immediately, but let the slope grow in and let the compaction set.

Exactly. In our new development area, a few have also been filled up – now they just have an even steeper slope from the end of the house to the end of the property – with us it’s almost straight down to the lower boundary, since we already go one story down due to the basement. BTW nobody waited there – the slab came 2-3 days later directly. Who knows how that will look in 10-15 years. The terrace will probably decouple from the house xD We save massive money on the outdoor facilities – the neighbors are just starting now ;) and there’s no doubt who has the nicer house on the slope.

I have no idea if and how it affects things when the ground settles by 1, 2 or even 5 cm or more in some places. It's simply my concern.

Nevertheless, the artificial slope remains. It can be made steeper than 30-35%. What the structural engineer says is a different matter. With our up to 42°, he complained massively. We have to be careful and never remove all the plants. Mind you, that is natural slope, nothing artificially created, and soil class 6. It will also never happen that everything is removed there all at once. It's a backbreaker to work there. I would never artificially create a steep slope for myself that has to be maintained. My parents indulge in the luxury and have the garden and landscaping contractor come around every 2-3 years.

You know that you can also use the basement for the 90k, right? :D
And cleverly hide heating etc., not on the ground floor?

I don't plan to accumulate that much junk to waste 90k on the necessary storage space ;-)
For heating, a utility room on the ground floor is enough – we're building single-story (that should be possible)

...I would never ever fill in a hillside lot. Such a shame :(

You don't have to. It's a matter of weighing options.