You’re talking about this navigation, I assume?
At the moment that’s controlled by a site setting called enable new user profile nav groups which we’ve enabled for everyone here on Meta. We intend on enabling this as the default on all sites, and it’s part of the changes we’ve made along with the addition of the sidebar.
The previous subnav for user pages created a bit of an awkward double-sidebar, which is the main reason behind the change:
