Sadly, today in 2018, the Context Help window decoration button is not very useful. Few KDE apps actually properly support the feature for mor than a tiny fraction of their UI, and no non-KDE apps do at all. So when you click on the button and mouse over various parts of an app's UI, you mostly see the red crossed our circle cursor. Not very useful.
We had a conversation about this during the VDG BoF at this year's Akademy and there seemed to be general agreement that this window decoration button is not very useful anymore. So let's remove it from the default set of window decoration buttons.