In the 4.x API there is no mention of a way to get a list of all widgets that have been added to the mapView via mapView.ui.add(); MapView only provides the notion of 'default' widgets via the ui.components property. Not sure why there is a distinction between default widgets and ones custom added, but there is, and it feels strange they are treated differently.
I need to implement responsive widgets and need a way to derive the active widgets list at any given time. I'm using a combination of default widgets and custom widgets. It appears I need to manage this list manually outside of the mapView instance?