7 files declare their use of EnhancerInterface in Drupal 9

EntityRevisionRouteEnhancer.php in core/lib/Drupal/Core/Routing/Enhancer/EntityRevisionRouteEnhancer.php

EntityRouteEnhancer.php in core/lib/Drupal/Core/Entity/Enhancer/EntityRouteEnhancer.php

FieldUiRouteEnhancer.php in core/modules/field_ui/src/Routing/FieldUiRouteEnhancer.php

FormRouteEnhancer.php in core/lib/Drupal/Core/Routing/Enhancer/FormRouteEnhancer.php

ParamConversionEnhancer.php in core/lib/Drupal/Core/Routing/Enhancer/ParamConversionEnhancer.php

ResourceVersionRouteEnhancer.php in core/modules/jsonapi/src/Revisions/ResourceVersionRouteEnhancer.php

RouteEnhancer.php in core/modules/jsonapi/src/Routing/RouteEnhancer.php