Thanks for the idea, but that won't work, sadly. It would show "projects" that are also subtasks, whereas I want "projects" which don't have an active subtask.
The closest to what I want is
Checked Off - no
Context is Project
Has Subtasks - no
But if it the "project" has a completed subtask, it won't appear on my search. My only workaround is to delete or remove completed subtasks from the parrent, but that's not so nice because it removes possibly useful reference to recently completed tasks.
What I'd really need is something like
Has subtask which is not checked off - no
or somehow the ability to generally search based on a property of possibly existing subtasks.