It is official now: every desktop needs to have a glass background!
Windows started this already with Aero.
macOS Big Sur will expect native apps to have a glass background
KDE apps are started to support a blurred background as well
Please note that this is different from a transparent or translucent background. Transparent just means you can see through the window, glass background will actually blur the contents behind the window. This is normally done via the Window Compositor but it needs to be enabled in order to get this effect.
For Qt on Windows, you can use the QtWin module to make a glass frame.
For KDE, there is no way to do this via Qt so you have to write a crazy xprop hack to enable the _KDE_NET_WM_BLUR_BEHIND_REGION flag.
As for macOS, you will have to write some Cocoa code to get that working.
Rather than write a hack for each platform, Qt should provide a simple Window flag like "WA_GlassBackground" to make this effect cross platform.
This is important because end users expect their desktop apps to look and feel like a native app rather than something foreign. Having this flag would make cross platform development much easier.