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.