Except that STOP is handled at the carrier level and isn't even returned to the sender. It's effectively a mandated block command.
https://www.cullenllp.com/blog/fcc-adopts-new-tcpa-opt-out-r...
Things may have changed since, but used to work sending 8m sms messages before breakfast every day to people who subscribed to news, sports scores, etc.
> NETWORK MSG: You replied with the word "stop" which blocks all texts sent from this number. Text back "unstop" or "start" to receive messages again.
I assumed it was from my carrier (T-Mobile in the US), but now I'm wondering, as I have gotten different replies from other numbers. Maybe it came from the sender's provider? Or is just misleading.