We have added the below text to all the SMS at the end.
STOP to opt-out. Msg&DataRatesMayApply
If the recipient replies with STOP then the corresponding contact will not be sent SMS anywhere from the platform(except OTPs and password verifications.)
Follow this link to check the countries who has Opt-out feature.
Applicable for Twilio Service alone and for other SMS service providers Opt-out is not required.