Global menus (primary links, secondary links) have been converted into blocks

In Drupal 7 and older, primary and secondary links (menus) have always been handled differently than other menus. Primary and secondary links are intended to display important links that are featured in themes. This led to an architecture supporting the primary and secondary links that became rigid and hardwired in its implementation, because it was closely intertwined with themes.

In Drupal 8, primary and secondary links have become unspecialized. They have been converted to blocks and are now managed just like any other block. A theme can still choose to provide "primary and secondary links" functionality, by adding the "Primary menu" (primary_menu) and "Secondary menu" (secondary_menu) regions. These regions are intended to be used by only a single block at a time, and only by menu blocks (just like the names suggest). This is what the default page.html.twig does (which custom themes inherit unless they overrides it) and what the default front-end theme — Bartik — also does.

API changes

• The primary and secondary menus are converted into blocks, and therefore no longer available as theme settings or as variables in the page template.
• The following API functions are removed:
  • function menu_main_menu()
  • menu_secondary_menu()
  • _menu_get_links_source()
  • menu_navigation_links()
• The primary and secondary menu settings are removed from menu_ui.settings and theme.settings.
• Two new regions in Bartik: primary_menu and secondary_menu.