ive seen it few days ago while using the installer on the netinstall iso.. its an option to import keys from github while creating the regular user...

When installing new system, there is an option to input your github/launchpad username

http://manpages.ubuntu.com/manpages/bionic/man1/ssh-import-i...

Where I see it is in the interactive installer for Ubuntu server, there's a screen that asks if you want to install SSH on the host and then it asks if you want to import keys from GH. You pass it a GH user name and it lists the keys to be imported.