'Noble Numbat' users will face a major post-install upgrade, which isn't ideal
Release Candidate 1 of Plasma 6 appeared last month and this gap in the KDE release cycle also gave developer Rafael Sadowski time to. Out-of-support or not, the last version of KDE 5 will be going into both the next OpenBSD and Kubuntu. This is hardly the ideal point in the life cycle of KDE 5 to use it in a long-term support Linux distro, but the Kubuntu team has little option. KDE 5.
from developer Nate Graham reveals, it's still about a month away. That doesn't leave enough time to integrate it with the Ubuntu 24.04 base OS, including testing and so on. Equally, a freshly released point-zero edition of such a large and complex desktop isn't an ideal match for a long-term distro release.
All the same, from the description of KDE 6 RC2, it sounds like the remaining issues are fairly modest. The finished 6.0 release will probably appear before Kubuntu 24.04, and users will be able to update to it. The precedingannounced that it's now possible to update the now two-year-old 22.04 to the latest Plasma 5.