isn’t standard, since knowing where and for how long to long-press (when using a tablet) or hover (when using a pointing device) isn’t whatsoever intuitive.
As an example, would you be aware, as a new user, that you can hover over some of these items to see tooltips?
As you point out, the pattern is somewhat new. I used it for the first time about four years ago. Previous to this it wasn’t a really a pattern KDE contributors were considering. Since then it was used more and more, but this is one of the reasons you aren’t seeing it in some places where it might make sense.
So now it is very easy to put such help into a button, but it still needs people to actually spend the time to do it. IMO it is very easy to do now even for total programming beginners. There are also currently new Human Interface Guidelines being written which will contain explicit advice on when to use this pattern, so new contributors wouldn’t even be left alone when it comes to the design aspect of this.
