Now "phonon4qt5-backend-gstreamer" is the next problem candidate for building Siduction ISOs.
It became unavailable and concerns several 'graphical' ISOs, KDE being one of them.
On the other hand there is now "phonon4qt6", but for that "phonon4qt6-backend-gstreamer" is (still?) 'unavailable', although 'suggested' by the package.
The KDE ISO seems to build by just removing/commenting out "phonon4qt5-backend-gstreamer", but I can not yet say, what consequences follow from that.
It appears that Qt5 is starting to become deprecated as of Plasma 6.6.4 as I notied for example Qt5 theming support is starting to be taken away. This is like when Qt4 was being removed from Plasma5 and Debian some years back.