Beside Neon and Kubuntu, are there other Linux distros that:
are based on Ubuntu (like the above and like Linux Mint or Elementary are), and
feature the KDE Plasma desktop?
After a lot of experience with many Linux distributions my general conclusion is that, in order for me to have a stable system (considering my hardware) I have to stay close to an Ubuntu base. I am very happy with the 18.04 base in fact, although 20.04 is fine too.
For me all rolling releases proved unstable (be they Arch-based like Manjaro, or Debian-based like MXLinux or SolydX), especially wifi and bluetooth stability was often dubious. Basic things that work out of the box in Ubuntu flavors or Mint take a lot of pain and uncertainty on other platforms, in my case. It is experience that pushed me into this corner, I guess the explanation for this is in the real improvements made by Ubuntu in the support for the machines I use. The same idea justifies the search for further improvements on the same solid base, and thus Linux Mint comes to mind: the "perfect" Linux for me is now Mint Xfce 19.2. It is both sad and comic and also significant for the Linux world that even an upgrade to Mint 19.3 Xfce brought this perfection to an end for me, not to mention upgrade to version 20. I see no problem in continuing with the 19.2 Xfce Mint, but I would like to try the Plasma desktop on a separate partition.
Before taking the obvious option of Kubuntu (be it 18 or 20 LTS; I have tried Neon, and it's a monstrous concoction from the point of view I've just emphasized) I want to know if there are other Linuxes that use Plasma on the Ubuntu base.