SQL*Plus 19.9.0.0 shows this syntax
Usage: CONN[ECT] [{logon|/|proxy}
[AS {SYSDBA|SYSOPER|SYSASM|SYSBACKUP|SYSDG|SYSKM|SYSRAC}] [edition=value]]
where <logon> ::= <username>[/<password>][@<connect_identifier>]
<proxy> ::= <proxyuser>[<username>][/<password>][@<connect_identifier>]