The HIG is kind of a mess
At the moment, our HIG mostly purports to describe reality, but it's not regularly updated, so reality drifts out of sync with it and what it recommends, so it become un-actionable. Many parts of it are also over-broad and generic, and not really actionable. There's a lot of filler and philosophizing.
Another complication is that we have QtWidgets and QtQuick apps, which often follow subtle different UI conventions whose differences arise because of divergent capabilities and ease of implementation on those platforms. For example, QtWidgets apps have customizable toolbars because they're easy to implement there, while our QtQuick apps don't because the feature is not implemented there. But our QtQuick apps make heavy use of placeholder messages because they're easy to implement there, while our QtWidgets apps largely don't because it's hard.
Plasma Mobile is also a space where there is heavy UI innovation and experimentation, much of which has no HIG guidance or even explicitly violates HIG guidance.
This is a non-ideal state of affairs, without easy paths forward. Let's discuss what we do about it.