The problem is that doesn't stay contained for those few users. If a certain number of users mark your confirmation email as spam, your reputation suffers and there's a higher likelihood that suddenly your emails get sent to spam for all other users, because of that reputation hit.

If sender reputation is tracked that way, maybe reporter reputation should be tracked the same way. If someone reports a legitimate confirmation mail that they signed up for, as spam, then it's the spam report that shouldn't be taken seriously.

Overeagerly classifying legitimate email that someone subscribed to as spam is no better than classifying spam as legitimate. Especially on a system like GMail where one person's incorrect classification would lead to other people not receiving their subscriptions in their normal mailbox.

Certainly.

But that is not how it currently works.

I’m continuously surprised to learn about all the little ways Google makes my internet experience worse, even though I don’t use their services.

Is there a reliable way to tell if an account is forwarding to Gmail? That way, you could reserve your spammer tools (not judging you for using them) for gmail users, and treat the rest of your subscribers with the respect you clearly intended.

Unfortunately I don't believe this is limited to Google. For example Yahoo lists the same best practices on their bulk sending best practices: https://postmaster.verizonmedia.com/best-practices