After installing plasma-pa you get the following error when trying to add it to the panel: qrc:/qt/qml/plasma/applet/org/kde/plasma/volume/main.qml:21:1: module "org.kde.plasma.private.volume" is not installed The error goes away if you restart plasma. It would be better if you didn't have to restart plasma.
Thanks for your report; it's valid, but only because the taskamanger queries for this import as an optional dep. Plasma-pa is considered part of the core. If a distro doesn't ship this by default, they've messed up. I don't want to spend time on this.
Afaict it is shipped as a package called plasma-pa in both Arch and Ubuntu and not installed by default.