Pinpoint SMS optout / opt back in

Yes, Only if you use Self-managed Opt-outs with Pinpoint. “By default, when a customer sends a message that begins with HELP or STOP to one of your dedicated numbers, Amazon Pinpoint automatically replies with a customizable message. In the case of incoming STOP messages, Amazon Pinpoint also opts the customer out of receiving future SMS messages. If you prefer to manage HELP and STOP responses by using a service other than Amazon Pinpoint, you can enable self-managed opt-outs.” This typically involves a Lambda that triggers a response instead of Pinpoint automatically responding.

https://docs.aws.amazon.com/pinpoint/latest/userguide/settings-sms-managing.html#settings-account-sms-self-managed-opt-out