18 files declare their use of RouteBuildEvent in Drupal 9

AdminRouteSubscriberTest.php in core/modules/system/tests/src/Unit/Routing/AdminRouteSubscriberTest.php

ContentModerationRouteSubscriberTest.php in core/modules/content_moderation/tests/src/Unit/ContentModerationRouteSubscriberTest.php

EntityResourcePostRouteSubscriber.php in core/modules/rest/src/EventSubscriber/EntityResourcePostRouteSubscriber.php

EntityRouteAlterSubscriber.php in core/lib/Drupal/Core/EventSubscriber/EntityRouteAlterSubscriber.php

EntityRouteProviderSubscriber.php in core/lib/Drupal/Core/EventSubscriber/EntityRouteProviderSubscriber.php

LayoutBuilderRoutes.php in core/modules/layout_builder/src/Routing/LayoutBuilderRoutes.php

LayoutBuilderRoutesTest.php in core/modules/layout_builder/tests/src/Unit/LayoutBuilderRoutesTest.php

ModuleRouteSubscriberTest.php in core/tests/Drupal/Tests/Core/EventSubscriber/ModuleRouteSubscriberTest.php

NoBigPipeRouteAlterSubscriber.php in core/modules/big_pipe/src/EventSubscriber/NoBigPipeRouteAlterSubscriber.php

ParamConverterSubscriber.php in core/lib/Drupal/Core/EventSubscriber/ParamConverterSubscriber.php

PathRootsSubscriber.php in core/lib/Drupal/Core/EventSubscriber/PathRootsSubscriber.php

PathRootsSubscriberTest.php in core/tests/Drupal/Tests/Core/EventSubscriber/PathRootsSubscriberTest.php

ResourceRoutes.php in core/modules/rest/src/Routing/ResourceRoutes.php

RouteMethodSubscriber.php in core/lib/Drupal/Core/EventSubscriber/RouteMethodSubscriber.php

RouteSubscriberTest.php in core/modules/views/tests/src/Unit/EventSubscriber/RouteSubscriberTest.php

Contains \Drupal\Tests\views\Unit\EventSubscriber\RouteSubscriberTest.

SpecialAttributesRouteSubscriber.php in core/lib/Drupal/Core/EventSubscriber/SpecialAttributesRouteSubscriber.php

SpecialAttributesRouteSubscriberTest.php in core/tests/Drupal/Tests/Core/EventSubscriber/SpecialAttributesRouteSubscriberTest.php

UserRouteAlterSubscriber.php in core/modules/serialization/src/EventSubscriber/UserRouteAlterSubscriber.php