Google has been rumored to deploy spam protection in Android's default Messages app for some time now, and it looks like that feature is finally going live, at least for some users. Not everyone has it yet, but with so many people starting to see it pop up it shouldn't take too long before it's available for all Messages users.
Once the server-side change happens for your device you'll get a notification that spam protection is now functioning on your phone. The app will then start notifying you if you get any messages from suspected spam numbers, and Google's support site lists how it handles all of this in a private way.
Essentially, Messages only views the number that sent you a text, but it doesn't examine your phone number or the message contents. On the backend Google can likely examine that number for bot-like behaviors (faster than usual messages, tons of messages in short time frames) and then issue that warning to you without needing to see any of the data on your device. If something slips through then you can manually report the number for spam which will help Google build up its database and ability to detect spam numbers.
Worst case scenario, if you're really uncomfortable with Google detecting any numbers that text you, you can simply disable the feature entirely.
via: Android Police
