According the console below the properties multipleSelectionEnabled and selectionEnabled of LayerList are deprecated on version 4.29
However, as of ArcGIS JS API 4.3, they are not:
https://developers.arcgis.com/javascript/latest/api-reference/esri-widgets-LayerList.html
Deprecated means they will be going away, typically a few releases after announced. to give users time to transition.
If you go to the property links, they are marked deprecated.
Space issue, not enough room in the property list for details.