It wasn't listed in tech specs, but it was never hidden or kept secret. It probably could have been more explicitly detailed, but in the FAQs for the Nest Secure it even tells the user:
> Can Nest Secure detect breaking glass? No. We’re working on bringing glass break detection to Nest Guard, the main hub of Nest Secure. Nest Detect, the open/close motion sensor, doesn’t have a microphone, so it can’t detect breaking glass. But its motion sensors can detect movement by intruders as well as when a door or window opens and closes depending on how it's installed.
> Can Nest Secure detect breaking glass? No. We’re working on bringing glass break detection to Nest Guard, the main hub of Nest Secure. Nest Detect, the open/close motion sensor, doesn’t have a microphone, so it can’t detect breaking glass. But its motion sensors can detect movement by intruders as well as when a door or window opens and closes depending on how it's installed.
https://nest.com/support/article/Frequently-asked-questions-...
This was listed before this big announcement.