Risposta migliore
Anyone can opt out by sending a message that is exactly a stop word: stop, cancel, unsubscribe and their equivalents. That marks the contact as opted out and cancels the automations running in that conversation. Sending start resumes.
Two details that matter for compliance and for debugging.
It has to be the whole message. "stop" opts out. "please stop sending me the morning one" does not, because the message is not exactly a stop word. This is intentional: matching stop anywhere would opt out anyone who wrote "I cannot stop using this". If you see complaints from people who thought they opted out, this is usually why, and a human should handle those in the Inbox.
Opt-out is enforced on send, not on build. An opted-out contact stays in your segments and counts in your audience, but broadcasts and automated messages skip them. That is why a broadcast can report a smaller delivered count than the segment size.
You do not need to add an unsubscribe step to your flows for this to work; it is always on. What you should add is a visible mention of it in any sequence that spans days, because a person who does not know the word to send will block you instead, and a block is much harder to recover from than an opt-out.