10 files declare their use of RequestContext in Drupal 9

IFrameUrlHelper.php in core/modules/media/src/IFrameUrlHelper.php

IFrameUrlHelperTest.php in core/modules/media/tests/src/Unit/IFrameUrlHelperTest.php

PathBasedBreadcrumbBuilder.php in core/modules/system/src/PathBasedBreadcrumbBuilder.php

PathValidator.php in core/lib/Drupal/Core/Path/PathValidator.php

PathValidatorTest.php in core/tests/Drupal/KernelTests/Core/Path/PathValidatorTest.php

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

RouterTest.php in core/tests/Drupal/Tests/Core/Routing/RouterTest.php

SiteInformationForm.php in core/modules/system/src/Form/SiteInformationForm.php

TrustedRedirectResponseTest.php in core/tests/Drupal/Tests/Core/Routing/TrustedRedirectResponseTest.php

UrlGeneratorTest.php in core/tests/Drupal/Tests/Core/Routing/UrlGeneratorTest.php