I'm quietly really pleased with having been the mover for this change in @fedify.
Too many ActivityPub servers have put usernames in Actor URLs and it creates a complete mess when wanting to allow users to change their usernames, and created other confusion when you had non-user Actors that you wish to federate.
Here's the PR that prompted the change: https://github.com/dahlia/fedify/pull/140
https://hollo.social/@fedify/01920e31-2b71-7d3e-9d05-f5a6f347598b
Relatedly, because I was sure we had documentation on this, but we couldn't find it (I asked at issue triage today), I've opened an issue to get a proper primer page for Actor URIs/IDs va usernames + URLs.
https://github.com/w3c/activitypub/issues/468
cc @fedify