1. Spam from people who are just brute-forcing the space of possible email addresses. In this case, yes, clicking unsubscribe gives them information. But I will say, this spam is generally pretty well taken care of by existing spam filters, so personally I'm not concerned about this.

2. Spam from people who got your email address via some interaction. Maybe you bought something from them, for example. This sort of spam, they already have your email address and know it works. The upside here is that if you've made a purchase from them, it's much harder for them to avoid regulation due to the money trail, so if you unsubscribe, they have to actually unsubscribe you. There are loopholes in regulation, however.

How strong is the assumption that all users have only one email address and will keep that address long-term? I remember hearing that one could keep getting AWS free trials simply by signing up with a new email address.

In apple mail, even if you turn it off it will happily load the remote images if you try to forward the email.

I found this particularly troublesome when I wanted to forward a phishing attempt to IT, and Little Snitch showed mail trying to connect to the phishing site.

(this was not on catalina, maybe it is fixed?)