In Twilio[1] a "STOP" reply is an automatic add to blocklist. Their user doesn't have control over how that is handled. I wouldn't be surprised if other providers have the same controls in place behind the scenes.

[1] https://help.twilio.com/articles/223134027-Twilio-support-fo...

But you still get a webhook notifying you they said stop.

So as far as telling whether it’s a legit number, the spammers still win.