plasma 5.4.3 updated today and as expected the ability to hide dropbox was
broken.
in
/usr/share/plasma/plasmoids/org.kde.plasma.systemtray/contents/ui/ConfigEntries.qml
the setting was
switch (index) {
so changing it to
switch (currentIndex) {
has allowed me to hide the icon again.
So far the icon has remained hidden across one reboot - but this alo sometimes
happened in 5.4.2 so I'll keep an eye on it for the next few days.