For KDE development I would recommend the KDE Neon developer edition as it has all the tools you need to get started.
KDE
KDE is an international technology team creating user-friendly free and open source software for desktop and portable computing. KDE’s software runs on GNU/Linux, BSD and other operating systems, including Windows.
Plasma 6 Bugs
If you encounter a bug, proceed to https://bugs.kde.org, check whether it has been reported.
If it hasn't, report it yourself.
PLEASE THINK CAREFULLY BEFORE POSTING HERE.
Developers do not look for reports on social media, so they will not see it and all it does is clutter up the feed.
I'm out of the loop, but I used to use Slackware -- largely because the distro didn't get mad when I just installed things from source directly onto the filesystem. No dependency tracking ;)
But I had a lot of experience before that with other distros.
Good choices are probable OpenSuse (tumbleweed), Arch, or Neon. Actually, I don't know the current state of Neon... is that still a thing?
Neon seems ideally positioned for KDE-specific development. Latest KDE frameworks but not necessarily the latest Ubuntu base.
While KDE neon is great for casual users, contributors will find KDE neon excels. Whether you are a coder, tester, artist, or do bug triage: KDE neon is the easiest way to work on the latest KDE software.
Fedora may also be worth consideration, though its association with Ret Hat could end up being a limitation.
All KDE applications are well integrated, with a similar look and feel and an easy to use interface, accompanied by an outstanding graphical appearance.
I'm on Arch and very happy with the experience