This is currently not on our roadmap. You can retrieve this value by making a call through the Graph API. If this is needed for your scenarios, please continue voting and we will review at a later date.Mike Rousos commented
If username is a method of authenticating users (along with email address), then it ought to be retrievable as a claim
Just to provide an update, we are close to launching a private preview. We are in the final testing stages for this feature. We will have another update in the next few weeks with instructions on how to join the private preview.Mike Rousos commented
This would be useful for some IoT scenarios that don't support interactive flows. Restricting the local accounts only would be fine.