I'm not so sure that developers of other desktop environments are highly attentive to Plasma's features and look.

But I do agree that it tends to see key features implemented well ahead of its biggest competitors. It also has really good built-ins, and imo something that doesn't get emphasized enough when discussing its configurability is how _usable_ its configuration interfaces are. Plasma has a much better story for configuring global keyboard shortcuts than macOS does, for example. I generally find the KDE conventions for organizing configuration options impressively orderly and discoverable, especially given how much functionality they expose.