This would be an extreme security violation. There was a time when we could trust most of the users on the internet, but that time has long gone.
Spammers would love to have that ability so that they could better direct their spam. Instead they often resort to old lists containing user-ids deactivated over a decade ago.
Crackers would love to have that ability so they could try to login to known accounts. Given that many people use extremely simple passwords, this would make their life so much easier.
A list of valid users would make social engineering a break-in so much easier. It is often still relatively easy.
However, if you have legitimate access to a server, there are a number of ways you can get a list of valid (if not necessarily active) userids.