6 files declare their use of RenderableInterface in Zircon Profile 8

Breadcrumb.php in core/lib/Drupal/Core/Breadcrumb/Breadcrumb.php

Contains \Drupal\Core\Breadcrumb\Breadcrumb.

ExampleRenderable.php in core/modules/system/tests/modules/twig_theme_test/src/ExampleRenderable.php

Contains \Drupal\twig_theme_test\ExampleRenderable.

Link.php in core/lib/Drupal/Core/Link.php

Contains \Drupal\Core\Link.

theme.inc in core/includes/theme.inc

The theme system, which controls the output of Drupal.

TwigExtension.php in core/lib/Drupal/Core/Template/TwigExtension.php

Contains \Drupal\Core\Template\TwigExtension.

TwigExtensionTest.php in core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php

Contains \Drupal\Tests\Core\Template\TwigExtensionTest.