The floor tiles have been removed, how to remove the putty?

I think the tile adhesive should be removed.

If the screed is fine, why should it be removed? What is your plan?

The screed will stay, you are right, it is in good condition. I naturally want to do as little work as possible. The tiles have been mostly removed, the rest of the tiles should not be a problem. However, there is a "build-up" on the screed. 1) A light brown leveling layer possibly to adjust to the level of the parquet in the adjoining room. 2) The actual tile adhesive in gray stripes. The first three pictures should show the situation clearly. I can provide further photos if needed. The plan: I would like to have click vinyl flooring installed for bathrooms. Possibly on an impact sound insulation layer.

Ah ok. So I would try the remaining leveling compound and the old adhesive as Nordlys suggested. If the height fits, it might also be worth considering only removing what comes off easily and then creating a flat surface for the vinyl with new leveling compound? You can try it, maybe it goes better than expected and everything can be removed easily. But if not, I would probably take the path of least resistance.

You will have to sand after removing and possibly re-spackle. It won't work without any dents anyway.

No, not that anyway. That's true.

Thank you for the help.
I have probably reached the end of my options here.

I will have to have the rest done.
Companies have effective milling machines that handle the tedious removal of the covering down to the screed. As soon as I have removed all the tiles and the wall-mounted toilet, I will get a quote from a tiler. He can then also apply the leveling layer at the correct height as well as lay the impact sound insulation and the click vinyl for bathrooms.

I will keep reporting.