SUMMARY It is not possible to hide Kaffeine from the taskbar. STEPS TO REPRODUCE 1. Double click on the Kaffeine systray icon. OBSERVED RESULT Nothing happens, Kaffeine remains on the taskbar. EXPECTED RESULT Kaffeine should be removed from the taskbar, its window hidden, and restored when the systray is double-clicked again. SOFTWARE/OS VERSIONS Linux/KDE Plasma: (available in About System) KDE Plasma Version: 5.17.5 KDE Frameworks Version: 5.70.0 Qt Version: 5.14.2 ADDITIONAL INFORMATION This bug was reported and supposedly solved in bug#370918, about 3 years ago.
Can confirm on KDE Neon 5.19, Plasma 5.19.5, Frameworks 5.73, Qt 5.14.2