.NET 9.0 - .NET Maui from Frame to Border
Since .NET 9 the compiler warns “Frame” is obsolete: “Frame is obsolete as of .NET 9. Please use Border instead.”.
Well, unfortunately, it is not possible to simply replace Frame with Border. Frame was somehow a little easier to use.
<Frame BorderColor="{AppThemeBinding Light=#e5e7eb, Dark=#374151}"
BackgroundColor="{AppThemeBinding Light=#ffffff, Dark=#111827}"
CornerRadius="12"
Padding="16">
</Frame>
Simply make a frame with rounded corners.
That's not “out-of-the-box” with Border.
So let's take a look at an example:

The source for this is easy:
<Frame Grid.Row="1"
Padding="16"
CornerRadius="12"
BorderColor="{AppThemeBinding Light=#e5e7eb, Dark=#374151}"
BackgroundColor="{AppThemeBinding Light=#ffffff, Dark=#111827}">
...
</Frame>
So let's convert:
<Border Grid.Row="1"
BackgroundColor="{AppThemeBinding Light=#ffffff, Dark=#111827}"
Padding="16"
Stroke="{AppThemeBinding Light=#e5e7eb, Dark=#374151}"
StrokeShape="RoundRectangle 12,0,0,12"
StrokeThickness="1">
...
</Border>

More Info
Border – .NET MAUI | Microsoft LearnCategories