When setting up a workflow you can set the trigger for the workflow as a contact opting out. You can use this to trigger any automation or remove them from specific workflows and add to others when the contact opts out. To do this you will select the Trigger for your workflow and scroll down to the bottom of the different triggers and select "YetiText: SMS Unsubscribed"
Once that has been selected you will then click the option "YetiText Phone Number" and select "is Known"
Once that has been applied and saved this workflow will now trigger when a contact opts out of receiving txts from your YetiText number.