The automation builder has a new person trigger: SMS Opted Out. Add it to
a workflow and its actions run the moment a contact replies STOP (or a
carrier reports an opt-out on your behalf) — add a tag, run another
automation, create a task, whatever you need.
It fires once, on the moment a contact’s status flips from opted in to opted
out. If they opt back in and later opt out again, the trigger fires again.
Whether the contact re-enters the automation depends on its re-enrollment
setting; by default a contact enters an automation only once. Opt-outs found in
message history imported from another system don’t fire it.
Nothing else changes about how opted-out contacts are handled: we still block
texts to them, exactly as before. This just gives you a way to react to the
opt-out itself.