KIconLoader::Panel is used as a maximum icon size for panel applets. This is problematic for the following reasons:
- It is not communicated to the user that this is a maximum size, and communicating it is somewhat challenging with the current UI
- Most panel applets no longer respect it, instead either setting a hardcoded maximum size (rare) or scaling with the panel and respecting its margins, so using it as originally intended no longer really works
- Since it's configurable, users can change it, see no visible effects, forget to change it back, and then complain when Kickoff's panel icon is too small if they make their panel thick
Even in its heyday, the only conceivable application of the maximum panel icon size being configurable separately from the panel thickness itself is to be able to have a thick panel with small icons in it. This functionality was already mostly implemented with @niccolove's margins separator feature, and anything beyond that seems like pointless over-configurability to me.
I'd like to propose that we deprecate this icon size, port any users of it to use KIconLoader::Enormous for their maximum size instead, and then remove the configuration UI from the Icons KCM.