One handle is enough
Every user in the JSON API now carries a single handle field, and who-to-follow lookups ask for handle instead of username. The duplicate is gone, with no compatibility shim.
Top storyTook the lead from Remotes learns to knock when the window is closed #1138. The API now returns only a handle. Old clients that ask by username get no error and fall back to plain suggestions, so they must switch or show wrong data. Held off 7 challengers since.

You curl a profile and the JSON comes back lighter. A user used to arrive twice, once as handle and once as username. Now there is one handle, and it is the same one the profile page shows in its URL. Commit authors, message senders, notification actors and who-to-follow entries all come back the same way.
Credit mnapoli, who swept the second name out of every corner of the API. Mention lists on commits, messages and previews now carry just the handle, its offset and its length, counted in characters with the @ included. The matcher still accepts handles up to 39 characters and skips anything inside code. Who-to-follow still hands you three picks and three spares, paged with round. Ask beside a profile with ?handle= and the people that profile follows come first.
The catch is the quiet part. The API checks only for handle now, so an old client still sending ?username= gets no complaint. It falls through to the plain picks, as if you had asked for nothing, and any client reading the old username field will simply find it missing.