Guest mode is one of the Chromecast's relatively unknown but very handy features. It uses location proximity and WiFi availability to allow guests to cast content to your speakers or TVs without logging in to your WiFi network, provided they have the specific PIN code for that device. Whether you're throwing a party or have temporary visitors and you don't want to bother with sharing WiFi access, you can simply give them a 4-digit code and let them choose the media that gets played. The feature launched in December of 2014 on Chromecasts and carried over to Google Homes — because they're essentially cast targets. However, it now seems to have disappeared from all devices but Chromecasts.
If you head over to the Google Home app and choose any cast device, you can tap on the top right settings cog to access its different options. Among those, underneath the Device info subsection, you may find Guest mode. That's where you can enable the feature and see the specific PIN code for each device.
Currently, the menu is available on Chromecasts, but it's not there on any Google Home or Nest speaker. We've verified this across different users and regions. It's also not there on any third-party Assistant speaker or display or Android TV, but I don't think it was ever available on most of these in the first place.
Left & Middle: Guest mode on a Chromecast. Right: No guest mode on a Google Home Mini.
The removal of the option doesn't seem to be a mistake either. Google's support page for Guest mode on Chromecasts is still live, but the one for the Nest/Home speakers has been removed. Its latest cached version is from October 30, 2019. Technically, the Home's settings are controlled by firmware updates, so there must have been an update that disabled guest mode for Google Homes around that time.