Twilio is sort of a dream for spammers, they'll just make new accounts on it and spam campaigns on those new accounts. Political organizations do it all the time, if you get on a list you're never getting off. Lookup the numbers sending to you (Twilio's own lookup tool works great for this) and it almost always comes back Twilio/Zipwhip.

I only recommend responding STOP to short codes since there's more investment and vetting on getting a short code. Carriers will intercept the request for TFN/local numbers sometimes but I don't really trust it. These numbers are all going to be spammers buying pools of numbers to churn and burn. They'll just import their list into a new account if it unsubs.

Oh and btw, it's actually easier now as a spammer to tell when numbers get burned. A few years back when the CTIA handover on regs happened (and sending costs went up) the carriers finally started to respond with the delivery status of the sent messages. Before this they didn't respond and you only knew your provider delivered the messages to the carrier, not whether the carrier delivered them to the handset.