This is an old one - see this post
- I was just wondering if "nearest future" was any closer now than it was 2.5 years ago?
I would point out that in that 2.5 years (at least) that it has not been implemented, no one has gotten around to updating the documentation to mention this fact - leaving it to developers to discover the hard way (and consequently wonder if they are doing something wrong).
I did not buy the full Editor API needed to work around the problem (as linked in the post linked above), I only bought the Core - according to the documentation it had all the functionality I needed ... but you can't believe everything you read. In fact the FlattenAllFields method is less misleading than the Export method that faces the same problem but contains considerably more detail in the documentation - rather strange for a function that doesn't actually do anything. But a lack of Export I could work around directly using other Core methods, lack of Flatten seems to be a more difficult problem (presumably one reason why it has not yet been implemented).