plasma use the icon's for the system tray from the plasma theme. when you look at gnome and xfce the system tray and panel icons come from the icon theme (/usr/share/icons/...). Most popular icon designers add support for system tray icons in there icon set it would be awesome if plasma can use this icons and only as backup the icons from the plasma theme were used. Not every icon designer want to make a new plasma theme only to have the right icons in the system tray. In addition since plasma 5 all kde applications use first the icons from system settings and if there isn't the needed icon the icon from the app source was used. why should there be a difference for plasma.
If I remove the icons folder from breeze/default, the icons from system settings were used. Problem: the system tray isn't monochrome any more. Solution: -symbolic or -panel suffix
Related to this one? https://bugs.kde.org/show_bug.cgi?id=369618
*** Bug 366729 has been marked as a duplicate of this bug. ***
In principle this request makes sense, and I'd like to move towards removing all icons from the Plasma theme and having everything always use icons from the icon theme. However a practical challenge here is that we currently use Plasma theme icons to force some large icons to be monochrome-only. For example, the Show Desktop widget's icon. If we always use icons from the icon theme, we'd need to make sure we don't lose the ability to do this, or else we'll be back to panel icons changing from monochrome to colorful based on the panel height. This seems quite unnerving to many users and we get bug reports about it.
*** Bug 451407 has been marked as a duplicate of this bug. ***
This has been basically done for Plasma 6; System Tray icons now come from the systemwide icon theme, not the Plasma style. If there is still demand for using special different icons just only the System Tray, we can implement a feature to use a separate KIconLoader instance that would then be able to use icons from a different icon theme for only the System Tray icons. See https://invent.kde.org/plasma/plasma-desktop/-/issues/82 for more information.