So far I can think of 2 (viable) types of safeguards against this:
1) Send follow-up email
@tobiaseigen’s solution of sending a follow-up email. In addition to what he already described, the follow-up email should only be send if the unsubscription happened when a user was not logged in. I hope there’s a way to differentiate between the two.
I think we’re talking about an edge case scenario here, so I strongly prefer the above solution. I’ll provide an optional one for good measure though.
2) Unsubscribe by email
If you’re not logged in when trying to unsubscribe, you will land on a Discourse page that says something like:
"You're not logged in. If you want to unsubscribe from {topic} / {category}, either [log in]() to change your notifications or <a href="mailto:unsubscribe@{forumdomain}?Subject=Unsubscribe me from {topic} / {category}" target="_top">click here to unsubscribe by e-mail</a>
Many mailing lists let users unsubscribe this way, although usually it’s as simple as unsubscribing from the entire mailing list and that’s that, as opposed to the more granular topics & categories.