Likely dependent on ESRI and the AGOL API, and not under Safe's control, but in some cases for example when migrating a hosted feature layer with multiple sub-layers I have wanted to control the layer IDs using unique layer ID assignment - similar to how it is done within ArcGIS Pro.
Note this is referring to the layer number e.g. s.arcgis.com/XTtANUDT8Va4DLwI/ArcGIS/rest/services/nz_regional_councils/FeatureServer/0 and not to an ID within an attribute, in case of confusion.
If it's not possible it's not possible, but it would be useful for sure. Have looked at the API and can't see a call to set this.
