You mark it as spam so that Google can add it to the metrics and stop allowing that account to spam everyone. If you just unsubscribe it only benefits you. If you flag it as spam it benefits everyone using the service.
Technically there’s no authority to determine if you ever did “opt in”, so that is a moot point. I get spam all the time claiming I opted in, when I know I’ve not. Nothing in an email is a guarantee, to include statements that you wanted this email or agreed to be contacted.
You mark it as spam so that Google can add it to the metrics and stop allowing that account to spam everyone. If you just unsubscribe it only benefits you. If you flag it as spam it benefits everyone using the service.
Technically it’s not considered spam if you opted in. Spam is unsolicited mail.
Technically there’s no authority to determine if you ever did “opt in”, so that is a moot point. I get spam all the time claiming I opted in, when I know I’ve not. Nothing in an email is a guarantee, to include statements that you wanted this email or agreed to be contacted.