7 files declare their use of MenuLinkInterface in Zircon Profile 8

MenuLinkDefaultForm.php in core/lib/Drupal/Core/Menu/Form/MenuLinkDefaultForm.php

Contains \Drupal\Core\Menu\Form\MenuLinkDefaultForm.

MenuLinkEditForm.php in core/modules/menu_ui/src/Form/MenuLinkEditForm.php

Contains \Drupal\menu_ui\Form\MenuLinkEditForm.

MenuLinkFormInterface.php in core/lib/Drupal/Core/Menu/Form/MenuLinkFormInterface.php

Contains \Drupal\Core\Menu\Form\MenuLinkFormInterface.

MenuLinkResetForm.php in core/modules/menu_ui/src/Form/MenuLinkResetForm.php

Contains \Drupal\menu_ui\Form\MenuLinkResetForm.

MenuTest.php in core/modules/menu_ui/src/Tests/MenuTest.php

Contains \Drupal\menu_ui\Tests\MenuTest.

menu_ui.module in core/modules/menu_ui/menu_ui.module

Allows administrators to customize the site's navigation menus.

SystemManager.php in core/modules/system/src/SystemManager.php

Contains \Drupal\system\SystemManager.