The CustomScatterSeriesDescriptor class is part of the Telerik.Windows.Controls.Spreadsheet assembly. This is why you don't find it when referencing the Telerik.Windows.Controls.Chart nuget package. The class is used with the charts feature of the RadSpreadsheet control, which uses RadChartView internally.
To achieve your requirement you can create your own custom descriptor with a custom class that derives from the ScatterSeriesDescriptor class and override its CreateInstanceCore method. Or alternatively, you can use the base ChartSeriesDescriptor as shown in the help documentation instead of creating a custom descriptor.
About the annotation not rendering, this happens because of the built-in UI virtualization of the chart and the fact that the chart doesn't know much about the size of the elements in the ContentTemplate of the custom annotation. There is no API that allows forcing the chart to draw elements outside of the viewport. To acheive your requirement give the approach with the bubble series a try.
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