I have a high DPI display. On X11, I scaled the display to 125% by forcing the font DPI to 120 (which is 96*1.25) as a workaround since Plasma's display scaling always caused slight blurriness of icons.
Since the upgrade to Plasma 6, this trick no longer worked and I got back the same problem of blurry icons, especially annoying in the taskbar.
Plasma 6 relies on Qt6, which changed the default scale rounding policy from Round to Passthrough. Apparently Round used to round down the 1.25 scale to 1 which practially disabled scaling for Qt, which lead to no blurriness. The Forced DPI then made fonts bigger without making icons bigger (which is ok, I can pick a larger icon in Dolphin for example)
To return to the old Qt5 behavior, add the following to .profile in your home:
export QT_SCALE_FACTOR_ROUNDING_POLICY=Round Source: Qt6 Documentation