This may be more general than just plasma... Plasma should not be editing it's own configuration files unless explicitly allowed to do so... so this means that plasma should check the permissions and ability to change the file( such as checking disk space if the file is writable) to make sure that it can actually write the file... and not allow users to change settings that are not allowed to be edited. This is a straightforward and clear thing... but someone went thru the trouble of making a dialog box popup saying that the configfile wasn't writable... so it might be an opportunity to refine the design. (I'm still rooting for KDE, windows10 has deleted enough of my work on it's own update schedule and disrespect, that I can understand that free software can have errors too) I made the plasmashellrc readonly in attempt to stop plasma from seemingly randomly changing my panel widths... it reset it on some vertical height changes... that's another severe bug... This one
See immutable entries: https://api.kde.org/frameworks/kconfig/html/options.html
Also note that while immutable config entries will cause them not to sync to disk when changed, Plasma will still happily apply any changes but then forget about them on next start. You might also want to try setting the shell "system immutable", cf [1] (the unlocked desktop one) which will disallow unlocking widgets, changing launchers, favorites, etc. [1] https://userbase.kde.org/KDE_System_Administration/Kiosk/Introduction#KDE_Action_Restrictions