There was much debate on this at AGMs some years ago.
In the end the compromise reached was that they can still be used, but in order to make them compatible with the "advisory route" thing, the org would have to tell people which leg of a particular route might contain a secret control.
One of the considerations was a certain ride which had a secret control which corresponded with an ambiguity in the route sheet, resulting in 90% of the field not passing (or knowing about) it, and a very lonely controller.