I feel like supporting “unsubscribe” via email is the first thing to build, this is something all mailing lists do … this is how I would build it:
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Support the magic, “unsubscribe” in title sent to a bare “reply by email address”. (ideally rate limited) Our email here is: reply+%{reply_key}@discourse.org … so emailing reply@discourse.org with the text “unsubscribe” will initiate unsubscribe sequence. We need to send a confirmation (at least optionally and default on) to protect against trivial email spoofing.
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Also trigger the unsubscribe sequence if anyone replies to any email with the text “unsubscribe”
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Have 1) and 2) default on, but can be disabled via a site setting
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Via a site setting allow “mailing list peoples” to add a a href to a mailto link for unsubscribe via email to all emails. Eg:
<a href="mailto:reply@discourse.org?subject=unsubscribe>unsubscribe from these emails</a>
This way none of this interferes or complicates existing stuff, it simply enriches the experience.
This is the same flow as unsubscribe via web would have anyway so the key here is building the “unsubscribe sequence” email.
The unsubscribe via web then simply becomes something akin to “forgot password” sequence where you enter an email and it triggers the unsubscribe sequence.
I would only deal with a global unsubscribe from absolutely everything case here. Not with granular changes.