That's intentionally undocumented; access to a contact's subscription management page is tightly controlled, and the mechanism is subject to change at any time. I'd advise against depending on that behavior going forward, since we use it internally and it's likely to change without notice.
You can create a static user-agnostic unsubscribe link by taking your primary HubSpot email domain and adding the following at the end: /hs/manage-preferences/unsubscribe-simple
We are also looking forward to the option to either subscribe users to email types via API or else generate an unsubscribe URL.
Since this post was in Aug 2016, is there a chance that the feature to generate an unsubscribe link has been implementd in HubSpot by now?
That’s a pitty. Together with the fact that it’s not possible to subscribe users to email types via the API (see Has anyone tried to build a custom email preference page?), this creates some pretty big user experience obstacles for us.
We don’t currently have a way to generate an unsubscribe URL for a contact. It’s something that we’re considering as something we’ll add in the future but there are no specific plans yet.