Google first introduced Guest Mode on Chromecast to let your friends easily cast from their phone to your TV just by entering a four-digit PIN code. Later on, it carried the feature over to Google Home speakers so that guests could easily play music too. However, while the feature remains on Chromecast, Google appears to have killed it on Google Home, according to Android Police. That means if your friend has Spotify and you don't, it could be a bit trickier for them to play music on your speaker.
The benefit of the feature was that guests didn't need to be connected to the same WiFi network as the speaker -- all they needed was a four digit PIN code. When Google first introduced the feature, it paired with devices using ultrasonic frequencies that could actually be heard by pets or people with good hearing.
Not only is the mode apparently disabled, but the the option to turn it on has disappeared from Home and Nest speaker device settings, as have the support pages for it. The instructions to make it work are still available on a cached page, however.
The feature is still available on Chromecast devices, so it'll still be easy to hook up that friend with the Netflix account. It's not clear why Google disabled it for Google Home, however, or exactly when the feature was disabled.