Modern versions of ssh support the "HashKnownHosts" file which prevents people from discovering what hosts you have ssh'd to in the past - and, as a side effect

Modern versions of ssh support the "HashKnownHosts" file which prevents people from discovering what hosts you have ssh'd to in the past - and, as a side effect, prevents tab-completion from discovering what hosts you have ssh'd to as well. ssh_config(5) says the option is disabled by default, but Ubuntu at least enables it in /etc/ssh/ssh_config.