The project (netcore3.1) is being packaged with a Windows Application Packaging Project (.wapproj + .appxmanifest) and published for sideloading using:
right click packaging project->Publish->Create App Packages
The error is: "It is not supported to build or publish a self-contained application without specifying a RuntimeIdentifier, Please either specify a RuntimeIdentifier or set SelfContained to false."
Other answers describe workarounds and methods to get the publishing system to recognise a RuntimeIdentifier, but none explains how to disable self-contained packaging. Surely there is a flag or property that can be set to create a package without the runtime libraries.