Afaik, it was possible with the Force Blur window script installed from System Settings, which was not ported to 6.
I tried GitHub - taj-ny/kwin-effects-forceblur: Blur any window on KDE Plasma 6 and it worked fine with on X11/X12 (=Wayland) windows, however was too buggy to be set to panels and right click menus (especially in comb. with Wobbly Windows). But you don’t have to as just blur behind windows is fine.
This one didnt work for me: