To opt out:

Discoverable server admins can opt out their servers or even individual channels by going to Server Settings, then Web Presence and choosing which channels you want to be visible. But, if you don’t change anything, public channels will have the setting enabled by default.

If you’re just a user in one of these huge servers, and you don’t want your messages to be viewable from Google, you can also opt your own messages out. Simply go to User Settings, then Data and Privacy and disable ‘Show my message content from Discoverable channels on web pages.’