Carriers and SMS service providers (like Twillio) obey that, no matter what service is behind.

There are stories of people replying STOP to spam, then never getting a legit SMS because the number was re-used by another service. That's because it's being blocked between the spammer and the phone.

STOP works thanks to the Telephone Consumer Protection Act (“TCPA”), which offers consumers spam protections and senders a framework on how to behave.

(Edit: It's relevant that STOP didn't come from the TCPA itself, but definitely has teeth due to it)

https://www.infobip.com/blog/a-guide-to-global-sms-complianc...

https://x.com/nikitabier/status/1867069169256308766

Again, no clue if this is true, but it seems plausible.