I agree, it's arguably clean, and definitely an improvement over Windows 98 era GUI, probably also XP era. But the world has moved on. Even KDE looks better today.
IIRC you can even configure custom colors, have dark mode, etc. It might look dated with the gray color scheme but that's really the only thing I can guess would look dated to someone.
:-o
I am honestly amazed.
IMHO KDE is fugly and it always has been.