9 files declare their use of RenderElement in Zircon Profile 8

ContextualLinks.php in core/modules/contextual/src/Element/ContextualLinks.php

Contains \Drupal\contextual\Element\ContextualLinks.

ContextualLinksPlaceholder.php in core/modules/contextual/src/Element/ContextualLinksPlaceholder.php

Contains \Drupal\contextual\Element\ContextualLinksPlaceholder.

ProcessedText.php in core/modules/filter/src/Element/ProcessedText.php

Contains \Drupal\filter\Element\ProcessedText.

RenderElementTest.php in core/tests/Drupal/Tests/Core/Render/Element/RenderElementTest.php

Contains \Drupal\Tests\Core\Render\Element\RenderElementTest.

ResponsiveImage.php in core/modules/responsive_image/src/Element/ResponsiveImage.php

Contains \Drupal\responsive_image\Element\ResponsiveImage.

TextFormat.php in core/modules/filter/src/Element/TextFormat.php

Contains \Drupal\filter\Element\TextFormat.

Toolbar.php in core/modules/toolbar/src/Element/Toolbar.php

Contains \Drupal\toolbar\Element\Toolbar.

ToolbarItem.php in core/modules/toolbar/src/Element/ToolbarItem.php

Contains \Drupal\toolbar\Element\ToolbarItem.

View.php in core/modules/views/src/Element/View.php

Contains \Drupal\views\Element\View.