I find that this would be a really helpful feature. I can think of many use cases, but will describe only one (different from the one above).
When making notes, I often like to have an ‘inset’ where I explore a matter, or describe an exercise. I would like to fold it this inset (transclusion) as it is not part of the main thread, so that it does not appear when I would like to focus on the main thread.
I have used headings to approximate this behaviour, but folding a heading also folds material below it until the next heading of an equivalent or higher priority.
A folding inset would be most useful in this regard, enhancing the clarity and succinctness of a note while still making available sub-threads from a different note. A folding inset that remembers its status across sync would be even better.