Does Bandwidth handle the ‘STOP’ keyword for Opt-Out?

Yes, but only on toll-free messaging. At the network layer, toll-free SMS enforces STOP commands. See the best practices guide for more information on how to implement. For local long code SMS and MMS, Bandwidth does not provide Opt-Out features. Click here for more information on how opt-out commands are handled on toll-free

However, the onus of honoring the STOP lies with the customer.