Additionally, the GUI in KDE plasma in System Settings is not entirely reliable. It sometimes makes stuff up about IPv6 rules for example. It seems to be a very light-weight wrapper over the FirewallD DBUS interface.

It’s in the announcement for Plasma 6.1, see https://kde.org/announcements/plasma/6/6.1.0/

To enable it, you need to use the Brightness & Colour widget. See also the merge request for this: https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/4093

I guess the documentation is a bit lagging still! I don’t know about a list of compatible keyboards, but I suppose you can just try it out to see if it works! 😁

Yes, but you have to enable the checkbox “Increase maximum volume” in the audio widget on the taskbar panel.