The thing is that I'm not experiencing this clipboard issue on Plasma, but on a fresh installation of Void Linux with niri. There are reports of this issue all over[1][2][3], so it's clearly not an isolated problem. The frustrating thing is that I wouldn't even know which project to report it to. What a clusterfuck.
I can't go back to X11 since the community is deliberately killing it. And relying on a fork maintained by a single person is insane to me.
[1]: https://old.reddit.com/r/hyprland/comments/1d4s9bw/ctrlc_ctr...
[2]: https://old.reddit.com/r/tuxedocomputers/comments/1i9v0n7/co...
[3]: https://old.reddit.com/r/kde/comments/1jl6zv7/why_does_copyp...
Re X11 maintenance... I think it's mostly "done" and doesn't really need a lot of work. So not sure I see a problem there.