I am looking for a stable Linux - I mean stable not necessarily because based on stable releases like Ubuntu LTS, but because well supported by maintainers and community — one that would be focused on Plasma/KDE desktop as their flagship DE in the way Ubuntu is on Gnome (and especially was on Unity), Mint (especially) on Cinnamon, Solus on Budgie, Elementary on Pantheon, Bodhi on Enlightenment, and not just a "flavor" in Ubuntu's sense, or a "spin" in Fedora's. Thus, although Kubuntu and Fedora KDE deserve a mention in this context, the question here is not about this type of distributions.

Plasma was always for me the most complex, interesting and intelligent desktop - one that it's worth an exclusive focus from the part of a Linux distribution.

What such distributions are there?

• And why not Neon?

Just read their FAQ:

What is KDE neon? KDE neon is a rapidly updated software repository. Most users will want to use the packages built from released software which make up our User Edition. KDE contributors and testers can use the packages built from KDE Git in the Testing and Unstable Editions. It uses the foundation of the latest Ubuntu LTS (20.04).

Is this "the KDE distro"? Nope. KDE believes it is important to work with many distributions, as each brings unique value and expertise for their respective users. This is one project out of hundreds from KDE.

...

Is it a distro? Not quite, it's a package archive with the latest KDE software on top of a stable base. While we have installable images, unlike full Linux distributions we're only interested in KDE software.

I had tested Neon and quickly found it one of the most unstable Plasma desktops, no matter the unshakable enthusiasm of adventurers who would quickly become bored with a stable Linux. (On my latest try it stopped working after changing the locale to "default" and logging out. That instability may be a freak event related to my hardware etc but it is significant). Neon is latest Plasma possible put on top of a stable Ubuntu and its purpose is that of testing the continuous changes of the desktop environment, and not that of building a KDE Linux distro. (There are indeed a lot of good reviews of how good and stable Nero is and I don't really know what to make of them, maybe people are pleasantly surprised by the stability after a day or a week, when they anticipated havoc...)