https://wccftech.com/disable-youtube-for-guests-on-google-home-rolling-out-to-some-users/

It would be absurd if Google Home devices didn’t have watertight integration with Google services.  You can use Google Home to send videos to a Chromecast device, tune in to a specific channel on YouTube TV, and play music with YouTube Music and a lot more. However, in its current state, it is easy for just about anyone to ask your Google Home speaker to perform some actions. Finally, Google is putting some restrictions in place to prevent such occurrences.

A new option was discovered in the Google Home settings by a Reddit user in the Google Home subreddit. There is a new toggle for “Disable YouTube for guests.” aimed at disabling the YouTube services for unrecognized voices. Although Google Home requires recognized voices for several actions, plenty of commands don’t. For example, anyone can ask about the traffic, but not everyone can add items to the grocery list.