Red Flag, this was an immediate change based on and in response to emerging insight and thus not subject to the normal documentation processes. I will pass your feedback on to the Autopilot team though for updates to existing documentation.
As for functionality, the only thing that truly changed is the username no longer being automatically populated in OOBE. Pre-assigning the user still works fine and still performs the same functionality that it always has: deploying user assigned apps to the assigned user during the technician portion of the Autopilot pre-provisioning process. Any other use was purely cosmetic and while we understand this may cause some disruption to your processes, this should be limited to aesthetics only with the user now having to type in their username.
Also, keep in mind that this is unrelated to the blog post and should be addressed through standard support channels if this is causing you an issue. You can post in Microsoft Q&A for further discussion, contact your account team, or open a support case.