The links may not even be on your site, I've just looked back on my ticket to dbseo on the issue back in 2016 and the "redirect-to" system creates an open redirect from any site it's enabled on - I did report it back then but they didn't do anything about it.
links like the following could be listed anywhere on the net and get indexed as from your site.
Code:
https://www.yourforumurl.com/redirect-to/?redirect=https://www.somepornsite.zzz/
You need to disable the feature and block /redirect-to/ in your htaccess and add a disallow in your robots.txt