1. In System Settings / Removable Media, the Defaults button has no effect on any settings, and the Apply button remains greyed out. 2. In Device Notifier Settings / Removable Devices, the Defaults button is permanently greyed out. The defaults on logging in after removing .kde/share/config/kded_device_automounterrc are "Enable automatic mounting of removable media" unticked and two greyed-out ticked settings, "Mount all removable media at login" and "Automatically mount removable media when attached".
Sorry that a response took over six years. Is this still happening for you? I tested the first point and it works for me without those problems.
On point 1, the same thing happens. I go to System Settings / Removable Storage (as it's called now). [Actually, I've just noticed it's called Removable Storage in the System Settings side menu, but Removable Devices in the module itself.] I click Defaults, but nothing changes, and Apply continues to be greyed out. I don't know what the defaults are, but if I make a change (say to disable "Enable automatic mounting of removable media" [called media now!] and click Apply, then click Defaults, then again nothing changes, and Apply continues to be greyed out. My expectation is that default settings would be made in the display, then the Apply button would make these default settings permanent. I'm using System Settings 5.18.4 under Kubuntu 20.04.
Thanks for checking again. It reminds me a bit of https://bugs.kde.org/show_bug.cgi?id=427494, maybe they are related.
I think they're different problems. In the case of the Default button not doing anything, my assumption is that the Apply button is greyed out precisely because nothing has changed (there's nothing to apply).
My speculation was that maybe there is something fundamentally broken for detecting deviating changes and the application of default settings. But the bugs might as well be unrelated.