Twitter does (though I don't know of anyone who has). As does Google and Facebook. Github too. Away from web services, gaming platforms like Nintendo's Switch

Twitter does (though I don't know of anyone who has). As does Google and Facebook. Github too. Away from web services, gaming platforms like Nintendo's Switch allow for user name changes. As does the more enterprisey databases like ActiveDirectory (Windows logins), ldap and Linuxes / Unix passwd table also allows for login name changes. To be honest, I've encountered more systems that do let you change your user name than systems that don't.