"This page assumes the status quo as of Plasma 5.7". Plasma is now on version 5.20 so the page you linked has some pretty old examples. The KDE folks are working pretty diligently to clean stuff like this up as they find it. Nate Graham's weekly blog documents this and other updates to the project.
Small half-joke: Only two words into the front page post I already hit a UX issue! (and a UI inconsistency)
> This week (KWin’s compositing code was almost totally rewritten...)
I then instinctively went on to the top of the article to check on the date, to get an idea of when was "this week" for the author... but there is no date. Oh, well :-)
Opening the post itself (instead of reading from the main page) does indeed change the UI and it now happens to show the date.
Loved the irony...
This all shows that even for people dedicated to it, getting UI/UX right is hard.
https://pointieststick.com/
I personally like KDE and it's Breeze Dark theme a lot.