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).

https://kaosx.us/packages/packages.php?sortby=name&sordir=asc&repo=kde-next

You can follow along there to see the progress. Milestones of something really working will be posted here.

demm changed the title to Start of Plasma & Frameworks Qt6 based .
10 days later

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).

5 days later

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:

4 months later

After a few months of quiet, work has resumed to get ready for Plasma 6.
Upstream now has master (in git) set to Frameworks 6 (Qt6 based thus) & Plasma master is in the proces of doing the same.
Now all in kde-next are build directly from master git, so Frameworks there is at version 5.240.0 & Plasma is at 5.27.80.
Not many KDE Apps can be build yet, since almost all still look for kf5 (eventhough that kf5 could be Qt6 based.....). Kate is one exception, it is up in kde-next as kate6 23.03.80.
Status image:

23 days later

All of Frameworks 6, Plasma 6 and a few more apps are now build against Qt 6.5.0. It is getting far more usable with apps like konsole, gwenview, spectacle and kate fully working. A preview (Live only) ISO with Plasma 6 will be released shortly, 2 images of the current status:

a month later

Falkon & Dolphin are now also ready for use as kf6/Qt6 based applications. Once Yakuake 'sees' Konsole again, instructions will be posted here to convert a regular (non-production) install from Plasma 5 to Plasma 6. It actually is quite easy to do, and you can also go right back to Plasma 5 should you no longer wish to use Plasma 6.
Images of Falkon & Dolphin kf6:

25 days later
17 days later

First two games Qt6/kf6 based have been added, KPat & Kmines.
KMines6:

Four more KDE Apps added (2 are dependencies/libraries), visible for users Kolourpaint6 & Kdeconnect-kde6.

11 days later

Haruna, Plasma-systemmonitor, Kwallet-pam are now also available as kf6/plasma6 apps, image is a Live session from a new Plasma 6 ISO created today:

16 days later

Octopi is now also Qt6/kf6 ready.
This time there was no upstream to use for porting, so the porting is done in KaOS (so far only KaOS, since Qtermwidget is about nowhere packaged for Qt6).
We'll see if upstream does want a Pull Request from this port 🙂

5 days later

Kdenlive Qt6/kf6 23.70.80 builds, but only runs with sudo so far:

8 days later

It seems that most KDE Apps are about ready to build with Qt6/kf6. Added the last few days are Skanpage, Khelpcenter, Kdeconnect-kde, Ark, Kdialog, Kjournald (not running yet, QML issue), image of a few of those new apps running from the ISO of today:

22 days later
9 days later

Calamares port to Qt6 builds & runs, though crashes on all QML modules and the partition module hangs on the requirements checker, but at least the port is now fully building (image shows the Welcome page as a widget, not QML):

22 days later

All in the kde-next repo moved to/has been rebuild (all latest git checkouts for kf6, plasma 6 and many of the KDE Apps as of Sept 29) on Qt 6.6.0RC, please note here if any breakage did occur because of the move to Qt 6.6.