Once a user disables forwarding or decides to stop using SIWA with an app, the emails will start to bounce.
#1, Having an account with a real email address is separate from the account they create when they use @privaterelay.appleid.com emails in SIWA. They will be completely separate accounts from the perspective of the developer.
#2, Turning off the flow of emails in iOS settings is not the same as clicking on an embedded link in email content to "Unsubscribe".
#3, Assuming that the "Unsubscribe" link embedded in the email is going to unsubscribe the @privaterelay.appleid.com email, then the developer would receive an unsubscribe email to whatever list processor they have and it would appear to be from the @privaterelay.appleid.com email address.
#4, The developer should remove the privaterelay email address from their list of outbound email contacts on their email server.