SUMMARY The massive amount of desktop effects are purely cosmetic changes, so I claim that they fit under the Appearance category. They do not change the behavior of the system in any meaningful way, and its counter-intuitive to place it there. I see the change to Plasma 6 as an opportunity, to change this.
Yes, this is something we do want to do, but it requires splitting the KCM in half--or more likely, deleting it and moving all of its various bits into other places. That's tracked already in https://invent.kde.org/plasma/systemsettings/-/issues/15.