The SelectionWidth of FeatureLayer seems to be "obsolete", however this example for 100.5 still uses it:
How to set SelectionWidth in ArcGIS Runtime 100.5 as it looks blurry?
I'm sorry for the confusion. I see that the samples documentation is not up to date.
FeatureLayer.SelectionWidth was deprecated at version 100.4 and has no effect at version 100.5.
At version 100.5, only the selection color can be configured, and it is set on the MapView or SceneView rather than individual feature layers.
MyMapView.SelectionProperties.Color = Color.Cyan;
That sample has been updated to use the new selection API, but the changes are currently reflected on GitHub only. I've logged an issue internally to update the samples documentation.
Edit: I did some more investigation and found this blog post which explains the reasoning behind the selection changes: https://www.esri.com/arcgis-blog/products/developers/announcements/selection-behavior-changes-in-run...