Panel started throwing verbose errors about missing plugins from the default configuration

The default configuration for xfce4-panel lists a few plugins that aren't part of its package. Prior to 4.19, the panel would semi-silently remove these plugins, logging only to the console, e.g.:

xfce4-panel-Message: 14:11:02.681: Plugin "(null)-7" was not found and has been removed from the configuration
xfce4-panel-Message: 14:11:02.682: Plugin "pulseaudio-8" was not found and has been removed from the configuration
xfce4-panel-Message: 14:11:02.685: Plugin "notification-plugin-10" was not found and has been removed from the configuration

However, in 4.19 panel started opening the verbose window about missing plugins instead. While I agree this behavior makes sense for existing installations, where user-selected plugins have disappeared for some reason. However, it seems sub-optimal for new installations, where the users are immediately welcomed by errors, that are apparently confusing to them.

Original bug report: https://bugs.gentoo.org/943519

Apparently, both pulseaudio and xfce4-notifyd plugins mentioned by the user were added to the layout back in 2019, in f586a3c9, with a note that they'll be "only added to the panel if available/installed".