11 files declare their use of SubscriberInterface in Simplenews 3.x

ConfirmMultiForm.php in src/Form/ConfirmMultiForm.php

MailEntity.php in src/Mail/MailEntity.php

Mailer.php in src/Mail/Mailer.php

MailerInterface.php in src/Mail/MailerInterface.php

RecipientHandlerAll.php in src/Plugin/simplenews/RecipientHandler/RecipientHandlerAll.php

RecipientHandlerSubscribersByRole.php in modules/simplenews_demo/src/Plugin/simplenews/RecipientHandler/RecipientHandlerSubscribersByRole.php

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

Hooks provided by the Simplenews module.

Subscriber.php in src/Entity/Subscriber.php

SubscriberExportForm.php in src/Form/SubscriberExportForm.php

SubscriptionManager.php in src/Subscription/SubscriptionManager.php

SubscriptionStorage.php in src/Subscription/SubscriptionStorage.php