I am answering my own question based on airblader's comment (thanks), just in case someone is interested in the future. the correct i3config for KDE looks something like this:
for_window [class="Plasma-desktop"] border none
for_window [class="Plasma-desktop"] floating enable
for_window [class="krunner"] border none
for_window [class="Kmix"] floating enable
for_window [class="Plasmoidviewer"] floating enable
for_window [class="Klipper"] floating enable, border none
You can also do this to start-stop plasma-desktop:
bindsym $mod+o exec plasma-desktop
bindsym $mod+p exec "killall plasma-desktop"
When plasma-desktop starts, a small background window will also start which you have to quit manually.
Did you use `xprop` on such a notification? Are you aware of the for_window directive?