Suggestion: Use HTTP 410 Gone #106
Description
An idea for your consideration: there are cases when an unsubscribe process does not work well (either due to client, or server, or both), and the subscriber continues to receive updates.
In this case, the subscriber might try to re-unsubscribe after receiving such updates, if the needed information to do so is available.
In FreshRSS, we return an HTTP/1.1 410 Gone status when we receive updates for which we have no subscription anymore, and would expect the publisher to cancel our subscription after receiving such a code.
https://github.com/FreshRSS/FreshRSS/blob/0bc59ba140b19d8e0a1762e5ffed66b0c61bd322/p/api/pshb.php#L57-L59
https://github.com/FreshRSS/FreshRSS/blob/0bc59ba140b19d8e0a1762e5ffed66b0c61bd322/p/api/pshb.php#L155-L157