KTooltipWidget wants to blur the background, we can't use KWindowEffects in KWidgetAddons a solution using the platform theme and properties was introduced.
Blurring is very much a property of the style rather than the widget itself. Especially when it can't be relied upon across desktop.
Given we have code to blur all other windows inside Breeze, it fits far more naturally there.
We can then drop the property change in KTooltipWidget in KF6.