21 files declare their use of Subscriber in Simplenews 3.x

ConfirmationController.php in src/Controller/ConfirmationController.php

Mailer.php in src/Mail/Mailer.php

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

Hooks provided by 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

SimplenewsSourceTest.php in tests/src/Functional/SimplenewsSourceTest.php

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

SimplenewsSubscriptionBlock.php in src/Plugin/Block/SimplenewsSubscriptionBlock.php

SimplenewsSynchronizeFieldsFormTest.php in tests/src/Functional/SimplenewsSynchronizeFieldsFormTest.php

SimplenewsSynchronizeFieldsTest.php in tests/src/Kernel/SimplenewsSynchronizeFieldsTest.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.rules.inc in simplenews_rules/simplenews_rules.rules.inc

Rules hooks for the Simplenews newsletter module.

SpoolList.php in src/Spool/SpoolList.php

SubscriberExportForm.php in src/Form/SubscriberExportForm.php

SubscriberMassSubscribeForm.php in src/Form/SubscriberMassSubscribeForm.php

SubscriberValidateForm.php in src/Form/SubscriberValidateForm.php

SubscriptionManager.php in src/Subscription/SubscriptionManager.php

SubscriptionsAccountForm.php in src/Form/SubscriptionsAccountForm.php

SubscriptionsBlockForm.php in src/Form/SubscriptionsBlockForm.php

SubscriptionsPageForm.php in src/Form/SubscriptionsPageForm.php