I can answer this. It’s very counter intuitive and somewhat limited, because you expect it to just be an option on the contact editing screen. It isn’t.
Instead, TO MANUALLY UNSUB, go to your contacts view and find the contact you want to sub or unsub, but DO NOT click on or edit the contact. Instead, select the checkbox to the left of the contact, then go to the top of the view and click the drop down arrow next to the multiple contacts selection box. You will have the option to flag all selected contacts as “Set Do Not Contact.”
TO MANUALLY RESUB an account after you’ve manually unsubed, find the contact and click on the name to display BUT DO NOT EDIT the contact. Over on the right, you’ll see a red “DO NOT CONTACT” banner with an “x” you can click to resub the account.
Is a user unsubbed using the unsub link or preference center, the “x” will not appear and you will not have any way to manually resub them except maybe to export, delete, and reinport the account.
My guess is the Mautic designers made it this way to cut down on abuse. Personally, as a long time software designer, I just don’t believe software should tell the human what they can and can’t do. Software is a tool to support the needs of the user and the user knows best. That said, I can’t really see a legitimate reason to manually resub someone unless you were the one who unsubbed them, so in this case it’s probably not a bad design–it’s just damned hard to find.