Old version man page:
Use non privileged port. With this you cannot use rhosts or rsarhosts authentications, but it can be used to bypass some firewalls that dont allow privileged source ports to pass.
Specifies whether to use privileged port when connecting to other end. The default is yes if rhosts or rsarhosts authentications are enabled.
Straight from the source code:
case 'P': /* deprecated */
options.use_privileged_port = 0;
It sets the option to 0, which seems to be the default value anyway, so it does nothing. Ignoring deprecated options helps backwards compatibility in cases where it does not affect functionality...
/* deprecated */ change dates back to 2002-09-04.
Even the bit of code I quoted above (from openssh-portable 7.7) just about got removed in a recent commit:
upstream: Deprecate UsePrivilegedPort
now that support for running
ssh(1) setuid has been removed, remove supporting code and clean up
references to it in the man pages
We have not shipped ssh(1) the setuid bit since 2002. If ayone
really needs to make connections from a low port number this can
be implemented via a small setuid ProxyCommand.
ok markus@ jmc@ djm@
It still ignores the option but no longer even sets an internal variable to go with it.