Plasma 5.27 will be the last Plasma version based on Qt5, so it is getting time to see how far Plasma Qt6 based will build. That means, all of the Frameworks will have to be build on Qt6 first.
Close to 1/3 of the Frameworks (5.99.0 based) build on Qt6 are now in the [kde-next] repo. Not installable alongside Qt5 based versions, since the libraries share the same name (header files are all KF6xxxx named).
You can follow along there to see the progress. Milestones of something really working will be posted here.
It shows projects still not compileable
Adding all the needed Frameworks (5.99.0) to build Plasma is completed.
A start is now made to add Plasma 5.26.3 Qt6/kf6 based packages, about a quarter of the Plasma packages is up.
Still not something usable.
One (test) install is converted to Plasma 6. SDDM shows, ksplash runs too, but no plasma desktop yet (plasma-desktop package fails to build so far, issue with incompatible kio 5.99.0 for needed latest plasma-desktop patches, kio 5.100 is needed).
Plasma 6 in virtualbox, limited of course, but it is up.
First application added to the Frameworks 6/ Plasma 6/ KDE apps builds in kde-next.
Konsole (Qt6 based), works good, so now it is possible to actually do something with a pure Plasma 6 install: