Display Name

[Tony Derricott]

Is it possible to remove the username (or make it show email instead of username) displayed under the Display Name when beginning a chat with a user?

[#2]

No, not a way to customize Teams at that level. However you can suggest the change on uservoice.

[#3]

Do you know if there's any reasoning documented anywhere as to why they chose to display username over email or nothing?

[#4]

No probably not other than internal design documents, that's a pretty minor UI item.