KDE 4.4ish (early 2010) was when it finally started to become stable enough to be mostly usable, so that's close enough to just saying "KDE 4".
Unfortunately it's looking like KDE 6 is going to be another catastrophic upgrade, unlike KDE 5 which I barely noticed. Both of my Debian bookworm->trixie upgrades had showstopper bugs that required the terminal to fix KDE, there is multi-second lag unless you turn off some of the new features, and significant uninvestigated breakage remains even after that.
"Plasma [i]s Hell" is well-named, not that it's the only problem.
Unfortunately it's looking like KDE 6 is going to be another catastrophic upgrade, unlike KDE 5 which I barely noticed. Both of my Debian bookworm->trixie upgrades had showstopper bugs that required the terminal to fix KDE, there is multi-second lag unless you turn off some of the new features, and significant uninvestigated breakage remains even after that.
"Plasma [i]s Hell" is well-named, not that it's the only problem.