SUMMARY

All entries on system tray have some visibility options (Always hidden, Shown when relevant, Always show...), when using multiple screens you can configure this independently on the different instances of  the system tray icon, personalizing your panel.

Default system entries have an option "Disable" that does so this entry is never shown, be it on the hidden or in the visible tray. However, application specific ones (i.e. octopi, discord...) do not have this option, it's not possible to disable those.

I'm aware it can be disabled on the application itself or set to always hidden, but those limit the options of multi-screen or even multi-widget configuration.

There is a discussion about this topic: https://discuss.kde.org/t/how-to-disable-specific-app-icon-on-system-tray/27077/9

STEPS TO REPRODUCE
1. Add an application with system tray icon (i.e. Octopi Notifier, Discord);
2. Enable it on the application configuration;
3. Try to change it's visibility to disabled on the system tray entries menu.

OBSERVED RESULT

It's not possible to disable application specific system tray entries

EXPECTED RESULT

Have the option to disable application specific system tray entries on the system tray.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 6.6.65-1-MANJARO (64-bit)
KDE Plasma Version: 6.2.4
KDE Frameworks Version: 6.8.0
Qt Version: 6.8.1

ADDITIONAL INFORMATION

I found similar bugs for system default apps, I think all arguments there also apply here too:
https://bugs.kde.org/show_bug.cgi?id=357081
https://bugs.kde.org/show_bug.cgi?id=423408