RevealBackgroundBrush is available for use in the operating systems
specified in the Requirements section. However, we do not recommend
its use as it may be altered or unavailable in subsequent versions.
I think this should be the reason why RevealBackgroundBrush disappeared in WinUI3.
In WinUI3, RevealBackgroundBrush is no longer supported, and there is no related content in the Microsoft.UI.Xaml.Media Namespace.
In the document of
RevealBackgroundBrushI think this should be the reason why
RevealBackgroundBrushdisappeared in WinUI3.