Can you walk us through what broad, overall goal you're trying to accomplish more generally? See Contributing Feature Requests and Describing Root Problems for help describing root problems. As those documents repeatedly emphasize, it's really important for us to understand root problems.

For example, you mention that you want to find subprojects for a project. We expect you can use the "Subprojects" tab on the project page to do that. Have you found that tab? If so, can you explain why that isn't a good solution for you, or why it doesn't solve your problem completely? If not, maybe there's something we could use to make it more obvious or easier to find?

My overall goal is to create a changelog automatically from the resolved tasks in Maniphest and I want to sort the changelog entries by category (like "GUI", "I/O", "Setup", "Documentation" etc.)

My approach was to define a project called "Category" and add the categories as subprojects.

I am currently developing a python script which collects all the tasks that are resolved via a Conduit API call (maniphest.query). During the implementation of the script I realized that I need to know if a project tag is a category tag or something else. And here I am...I was a little bit surprised that the result of "project.query" does not contain any relationship information. As you mentioned earlier, there is a subproject tab on the project page, so obviously there are information about the relationship available.

Introducing Maniphest custom fields is not an option because we have more than one of our software products in Phabricator and the categories vary from product to product.

Writing these lines I think about adding an hash tag to each subproject. Something like "category_tag" should do the trick. But the question remains, why the relationship information is not part of the result of the API call (project.query)