7 files declare their use of RenderableInterface in Drupal 8

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

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

FormattedDateDiff.php in core/lib/Drupal/Core/Datetime/FormattedDateDiff.php

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

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

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