Hi Hüseyin,
The Projects API is indeed a bit old-fashioned in SOAP (almost 5 years old). I assume Mendix has a REST service on its backlog, but it surely hasn't have a high prio, as this SOAP version is still workable.
Depending on what you need, this SOAP can be used. If you download the msd-file referred to on the documentation page and rename it to zip, you can find the wsdl documentation right in it.
Good luck,
Johan