BuddyPress 3.0 and the template pack you have to choose

3.0 ships a second template pack alongside the original, and an existing site keeps the old one — so the new markup, which is the whole point of the release, is opt-in and easy to miss.

// Settings → BuddyPress → Options → Template Pack
// or in code:
bp_update_option( '_bp_theme_package_id', 'nouveau' );

// and the template hierarchy, which is where overrides go
// wp-content/themes/your-theme/buddypress/members/single/home.php

Nouveau uses cleaner markup and a proper JavaScript layer where legacy used inline scripts, and switching an existing site will break every template override that was written against the old structure. That is the migration, and it is genuinely a project on a customised community site. Starting new work on nouveau and leaving existing sites on legacy is the pragmatic position, with the same caveat as always about deferrals that never get scheduled.