11 files declare their use of LayoutDefault in Drupal 8

BlankLayout.php in core/modules/layout_builder/src/Plugin/Layout/BlankLayout.php

FieldLayoutBuilderTest.php in core/modules/field_layout/tests/src/Unit/FieldLayoutBuilderTest.php

Layout.php in core/lib/Drupal/Core/Layout/Annotation/Layout.php

LayoutBuilderTestPlugin.php in core/modules/layout_builder/tests/modules/layout_builder_test/src/Plugin/Layout/LayoutBuilderTestPlugin.php

LayoutDefaultTest.php in core/tests/Drupal/Tests/Core/Layout/LayoutDefaultTest.php

LayoutPluginManagerTest.php in core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php

LayoutTestDependenciesPlugin.php in core/modules/system/tests/modules/layout_test/src/Plugin/Layout/LayoutTestDependenciesPlugin.php

LayoutTestPlugin.php in core/modules/system/tests/modules/layout_test/src/Plugin/Layout/LayoutTestPlugin.php

MultiWidthLayoutBase.php in core/modules/layout_builder/src/Plugin/Layout/MultiWidthLayoutBase.php

TestLayoutContentFooter.php in core/modules/field_layout/tests/modules/field_layout_test/src/Plugin/Layout/TestLayoutContentFooter.php

TestLayoutMainFooter.php in core/modules/field_layout/tests/modules/field_layout_test/src/Plugin/Layout/TestLayoutMainFooter.php