>That's your local account, not information that is visible to anyone outside of your network.

There are a lot of unix tools to ask networks to query that information and they'll happily do it for other unix systems they connect to. When I was in college in the last 90s, I'd often lookup phone numbers and such of students at other universities using command line unix utilities.

Like what? I'm not familiar with any daemons that expose this info on a network interface. Maybe identd? AFAIK that can expose the username, none of the other fields, but is typically set to an alternate identifier.

Or do you mean shared mainframes with many users on the same system? That'd still be local to the system, you just happened to have access to that system.

Are you sure you're not just thinking of ldap/directory lookups?

Finger is one that was commonly available on university unix systems. You could do finger username@node.domain, and the username was often their email address or some common shortening of their name.