Another possible solution would be a query option for projects this you can control if you all projects (including subprojects) or just top level projects.

This request has the same design issues that showing all levels on a project's tag. Given both the number of levels (16) and the number of direct children (99+).

Problem: While this is technically correct, it is confusing,

Not sure if this task was meant as feedback on Projects (if so we can just merge it into the Erratta task), Or as an actual feature request (if so it will need to provide a more complete root problem).

There is no direct path that will work for all installs and not be likely janky UI-wise. For example you might be a member of a project 4 levels down, what are you expecting to see? The best I can come up with is a "only parents" checkbox on the search query, which would return only projects that are parents, and then group their direct children.

Root problem (a) is that the query lists sub-projects which I am a member of, and additionally it lists the parent project, which I am not directly a member of, but only through the sub-project. This is confusing, because I am listed as "joined" to project which I never really joined.
Root problem (b) is that several projects in the list might logically belong to a parent project, but are not shown in this way. This is confusing in general, because (1) a project's sub-projects are scattered all over the list, making it difficult to grasp as a whole, and (2) the correct interpretation of a project's name might depend on its parent project.

Root problem (a) is that the query lists sub-projects which I am a member of, and additionally it lists the parent project, which I am not directly a member of, but only through the sub-project. This is confusing, because I am listed as "joined" to project which I never really joined.

Generally this isn't a root problem of a feature request. It's just general feedback which is covered under T10349 (subproject UI concerns). Describing Root Problems and Contributing Feature Requests goes into fairly verbose depth on what we look for here.

Maybe a better way to think about "feature requests" are that there is a lack or inefficient means of solving a problem using Phabricator. So then the core problem might be "I use Projects as a means of checking in on things I am directly responsible for, but with subprojects it now lists both the project I joined and the parent project I inferred membership from. I have no means of filtering out inferred memberships". This is a root problem, and has a reasonably clear means of us thinking about and building a solution (maybe adding additional search options).

However, the problem you noted above is ambiguous as to the actual root problem, the one you encountered to first open the ticket. A simple solution to that as presented might be to change "Projects I Joined" label to "Projects I am a Member of". Maybe that still solves the problem? It's difficult for us to tell.

Personally I would probably want a means of filtering out inferred memberships, but maybe it sounds like you're OK with them, and just want inferred projects labeled as such?

In T10429#167793, @chad wrote:

Personally I would probably want a means of filtering out inferred memberships, but maybe it sounds like you're OK with them, and just want inferred projects labeled as such?

I was thinking the same, but thought I should rather not suggest a solution, but try to describe the problem more verbosely.

Thus: That solution sounds very good to me to improve situation (a).

We are planing to use subprojects in two variants:

1. As simple tags to organize different aspects of the parent project
2. As real projects to organize different products of an account

Further we are using a projects query on our start page. For the first scenario it would be helpful to see only the parent projects. In the second case it would be helpful to see each subproject I'm a member of but with the information which parent(s) the sub project has.

Personally I think the "only parents" checkbox for the projects query would be already very helpful.