Comment 11 for bug 1448512

Support for legacy (xembed) systray icons has been restored in Plasma workspace 5.5:

http://blog.davidedmundson.co.uk/blog/xembed_back

"In retrospect, I think we underestimated the fallout this would cause. [...] There's nothing wrong with changing our minds in response to user feedback."

Kubuntu 16.04 LTS is going to ship with Plasma 5.5 or newer, so this issue should be resolved then. Backports to earlier releases are unlikely, from what I've read.