15 files declare their use of Newsletter in Simplenews 3.x

ConfirmationController.php in src/Controller/ConfirmationController.php

RecipientHandlerManager.php in src/RecipientHandler/RecipientHandlerManager.php

simplenews.api.php in ./simplenews.api.php

Hooks provided by the Simplenews module.

simplenews.install in ./simplenews.install

Install, update and uninstall functions for the simplenews module.

simplenews.module in ./simplenews.module

Simplenews node handling, sent email, newsletter block and general hooks.

SimplenewsAdministrationTest.php in tests/src/Functional/SimplenewsAdministrationTest.php

SimplenewsI18nTest.php in tests/src/Functional/SimplenewsI18nTest.php

SimplenewsSendTest.php in tests/src/Functional/SimplenewsSendTest.php

SimplenewsSubscribeTest.php in tests/src/Functional/SimplenewsSubscribeTest.php

SimplenewsTestBase.php in tests/src/Functional/SimplenewsTestBase.php

simplenews_demo.install in modules/simplenews_demo/simplenews_demo.install

Simplenews_demo base install file.

simplenews_rules.module in simplenews_rules/simplenews_rules.module

Module file for Simplenews rules integration.

SubscriberMassSubscribeForm.php in src/Form/SubscriberMassSubscribeForm.php

SubscriptionManager.php in src/Subscription/SubscriptionManager.php

SubscriptionsFormBase.php in src/Form/SubscriptionsFormBase.php