14 files declare their use of RenderElement in Drupal 9

BreakLockLink.php in core/lib/Drupal/Core/TempStore/Element/BreakLockLink.php

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

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

form.inc in core/includes/form.inc

Functions for form and batch generation and processing.

LayoutBuilder.php in core/modules/layout_builder/src/Element/LayoutBuilder.php

media.module in core/modules/media/media.module

Provides media items.

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

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

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

StatusReportPage.php in core/modules/system/src/Element/StatusReportPage.php

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

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

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

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