As someone who has used KDE 3, 4 5 I would say the only really big change was 3 -> 4 and many aspects of it have been refined/improved. I think if you plunked someone down who had only used KDE 4, released in 2008, in front of the current version in 2021 they would require minimal learning to catch up.

Growth requires change and if you don't have a billion dollars up front you do this iteratively in small pieces by necessity.