Which heating pipes are used nowadays?

So I checked, these are welded steel tubes, outer diameter is 17.2mm. So 3/8 inch.

You can also simply use a Gebo fitting, then you don't need to cut any threads.

What kind of pipes are they and why should they be removed?

Where the radiator is connected, there is a thread! Definitely
Depending on where it is, it can still be used further.
In my case, the old steel pipes were not welded but everything was screwed. I'll see if I can find more pictures.

Sorry for only getting back to you now. I had a lot to do. The pipes are to be removed because they are now 51 years old. I did not want to simply extend them as shown in the pictures. All the lines run along the ceiling in the basement, so I don't have to chisel any walls. The niches are closed, and soon new holes will be drilled downward for the new copper pipes. These will then be laid under the ceiling parallel to the steel pipes and then connected very close to the heating system to the old steel pipes.

You can definitely do that. It's just really unnecessary.

Why should that be unnecessary?