Haha. I sent them an opt-out notice by email, and it bounced!
They are using Google email servers for discord .com and Google has apparently shadowbanned me. It gives an error message saying "The account [my email address] is disabled." but I have never created a Google or Gmail account, and my email address is on a domain not associated with Google at all.
So I've completed my obligation to opt-out. Discord will have no record of it, but I have the email server logs to prove I sent it.
If, in the future, anyone needs to sue Discord and forgot to opt-out, feel free to use this same excuse.