After the latest round of updates I no longer have the pidgin, steam, prime-indicator or dropbox icons showing in my system tray. I've verified that they are set to 'Shown'. 'Always show all entires' also doesn't show them.
EDIT: The latest updates fixed the problem.
Was that the update to Frameworks 26?
Yes, I'm on Frameworks 26 and I believe this started occurring after the update to Frameworks 26.
After I performed the upgrade I still can see pidgin (though it has a different icon now) and skype icon also appears.
Did you check if you really upgraded all packages ?
Maybe running a sudo apt-get update && sudo apt-get dist-upgrade might get you even some more updates that will fix it for you.
I tried running the dist-upgrade but it didn't pick anything up. Any non-KDE system tray icon is missing.
thats really odd, it seems to works fine here after updating to frameworks 26. I still see pidgin, skype, gmusicbrowser and vokoscreen.
Is the process xembedsniproxy running for you ?
xembedsniproxy is running for me. I tried killing the process and running it from the terminal to see what errors I would get. I get the following:
/usr/bin/xembedsniproxy
kde.xembedsniproxy: starting
kde.xembedsniproxy: Manager selection claimed
kde.xembedsniproxy: trying to dock window 96469382
kde.xembedsniproxy: adding damage watch for 96469382
kde.xembedsniproxy: Skip transparent xembed icon for 96469382 "Pidgin"
kde.xembedsniproxy: No xembed icon for 96469382 "Pidgin"
Unfortunately that is the entirety of the output.