As has already been said, this is a development step/tool, testing the waters to see what is missing to build against Qt6, things that need to be ported, things that might need to get implemented upstream ...etc.
Note that this isn't for general consumption yet, and that the KF libs so version is still 5 (it was discussed in the KF6 weekly meeting and the conclusion was that it would be too much hassle to switch this now before branching KF6), so it's strongly advisable to install this in a separate prefix from the rest of the system.
As part of this work, all Frameworks are being built with all deprecated KF code disabled (-DEXCLUDE_DEPRECATED_BEFORE_AND_AT=5.90.0 -DKF_DISABLE_DEPRECATED_BEFORE_AND_AT=0x055a00).
It's advisable to build them in the same order as shown by kdesrc-build --list-build frameworks, if module A depends on module B, B will be listed before A (but it's not a definitive order, i.e. some Tier1 frameworks are listed way down the list if nothing depends on them in KF ...etc), i.e. you have to build the whole stack from the bottom up so that it all links against Qt6 libs (won't build otherwise).
First the Frameworks that can already be built with Qt6 by passing -DQT_MAJOR_VERSION=6 (@mlaurent made a huge chunk of them). For MR's that haven't been merged yet, links are posted.
Done (or with an open MR):
kcoreaddons
ki18n
karchive
kconfig
kdoctools
kwidgetsaddons
kcodecs
kauth
kdbusaddons
kguiaddons
kconfigwidgets
kitemviews
kwindowsystem
kiconthemes
kcompletion
kcrash
kservice
sonnet
attica
breeze-icons
kglobalaccel
ktextwidgets
knotifications
kxmlgui
kbookmarks
kjobwidgets
kwallet
solid
kio
kparts
kpackage
kactivities
kdeclarative
kidletime
kunitconversion
syntax-highlighting
kdnssd
kitemmodels
syndication
threadweaver
kded
kfilemetadata
knewstuff
kpty
baloo
bluez-qt
kactivities-stats
kcalendarcore
kcontacts
kdesu
kholidays
kimageformats
knotifyconfig
kpeople
kplotting
modemmanager-qt
networkmanager-qt
prison
ktexteditor
krunner
oxygen-icons5
kdav
kwayland
frameworkintegration
kirigami
kquickcharts
purpose
plasma-framework
qqc2-desktop-style
Non-KF:
polkit-qt-1 (done)
packagekit-qt
appstream-qt
dbusmenu-qt
No action needed:
kapidox
Deprecated Frameworks:
kdelibs4support
kdewebkit
khtml
kjsembed
kmediaplayer
kinit
kjs
kross
kdesignerplugin
kemoticons
kxmlrpcclient